diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
index 36e64c9d6a2..e29848c187c 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java
@@ -669,7 +669,35 @@ public class OpenAPINormalizer {
     }
 
     /**
-     * If the schema is oneOf and the sub-schemas is null, set `nullable: true` instead.
+     * Check if the schema is of type 'null'
+     * 
+     * Return true if the schema's type is 'null' or not specified
+     * 
+     * @param schema Schema
+     */
+    private boolean isNullTypeSchema(Schema schema) {
+        if (schema == null) {
+            return true;
+        }
+
+        if ((schema.getType() == null || schema.getType().equals("null")) && schema.get$ref() == null) {
+            return true;
+        }
+
+        // convert referenced enum of null only to `nullable:true`
+        Schema referencedSchema = ModelUtils.getReferencedSchema(openAPI, schema);
+        if (referencedSchema.getEnum() != null && referencedSchema.getEnum().size() == 1) {
+            if ("null".equals(String.valueOf(referencedSchema.getEnum().get(0)))) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * If the schema is oneOf and the sub-schemas is null, set `nullable: true`
+     * instead.
      * If there's only one sub-schema, simply return the sub-schema directly.
      *
      * @param schema Schema
@@ -680,35 +708,19 @@ public class OpenAPINormalizer {
             return schema;
         }
 
-        if (schema.getOneOf() != null && !schema.getOneOf().isEmpty()) {
-            for (int i = 0; i < schema.getOneOf().size(); i++) {
-                // convert null sub-schema to `nullable: true`
-                if (schema.getOneOf().get(i) == null ||
-                        (((Schema) schema.getOneOf().get(i)).getType() == null &&
-                                ((Schema) schema.getOneOf().get(i)).get$ref() == null)) {
-                    schema.getOneOf().remove(i);
-                    schema.setNullable(true);
-                    continue;
-                }
+        List<Schema> oneOfSchemas = schema.getOneOf();
+        if (oneOfSchemas != null) {
+            if (oneOfSchemas.removeIf(oneOf -> isNullTypeSchema(oneOf))) {
+                schema.setNullable(true);
 
-                // convert enum of null only to `nullable:true`
-                Schema oneOfElement = ModelUtils.getReferencedSchema(openAPI, (Schema) schema.getOneOf().get(i));
-                if (oneOfElement.getEnum() != null && oneOfElement.getEnum().size() == 1) {
-                    if ("null".equals(String.valueOf(oneOfElement.getEnum().get(0)))) {
-                        schema.setNullable(true);
-                        schema.getOneOf().remove(i);
-                        continue;
+                // if only one element left, simplify to just the element (schema)
+                if (oneOfSchemas.size() == 1) {
+                    if (Boolean.TRUE.equals(schema.getNullable())) { // retain nullable setting
+                        ((Schema) oneOfSchemas.get(0)).setNullable(true);
                     }
+                    return (Schema) oneOfSchemas.get(0);
                 }
             }
-
-            // if only one element left, simplify to just the element (schema)
-            if (schema.getOneOf().size() == 1) {
-                if (Boolean.TRUE.equals(schema.getNullable())) { // retain nullable setting
-                    ((Schema) schema.getOneOf().get(0)).setNullable(true);
-                }
-                return (Schema) schema.getOneOf().get(0);
-            }
         }
 
         return schema;
@@ -726,34 +738,18 @@ public class OpenAPINormalizer {
             return schema;
         }
 
-        if (schema.getAnyOf() != null && !schema.getAnyOf().isEmpty()) {
-            for (int i = 0; i < schema.getAnyOf().size(); i++) {
-                // convert null sub-schema to `nullable: true`
-                if (schema.getAnyOf().get(i) == null ||
-                        (((Schema) schema.getAnyOf().get(i)).getType() == null &&
-                                ((Schema) schema.getAnyOf().get(i)).get$ref() == null)) {
-                    schema.getAnyOf().remove(i);
-                    schema.setNullable(true);
-                    continue;
-                }
-
-                // convert enum of null only to `nullable:true`
-                Schema anyOfElement = ModelUtils.getReferencedSchema(openAPI, (Schema) schema.getAnyOf().get(i));
-                if (anyOfElement.getEnum() != null && anyOfElement.getEnum().size() == 1) {
-                    if ("null".equals(String.valueOf(anyOfElement.getEnum().get(0)))) {
-                        schema.setNullable(true);
-                        schema.getAnyOf().remove(i);
-                        continue;
-                    }
-                }
+        List<Schema> anyOfSchemas = schema.getAnyOf();
+        if (anyOfSchemas != null) {
+            if (anyOfSchemas.removeIf(anyOf -> isNullTypeSchema(anyOf))) {
+                schema.setNullable(true);
             }
 
             // if only one element left, simplify to just the element (schema)
-            if (schema.getAnyOf().size() == 1) {
+            if (anyOfSchemas.size() == 1) {
                 if (Boolean.TRUE.equals(schema.getNullable())) { // retain nullable setting
-                    ((Schema) schema.getAnyOf().get(0)).setNullable(true);
+                    ((Schema) anyOfSchemas.get(0)).setNullable(true);
                 }
-                return (Schema) schema.getAnyOf().get(0);
+                return (Schema) anyOfSchemas.get(0);
             }
         }
 
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java
index a51c30f2ce1..28f6abf540e 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/OpenAPINormalizerTest.java
@@ -141,11 +141,11 @@ public class OpenAPINormalizerTest {
         OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_0/simplifyOneOfAnyOf_test.yaml");
 
         Schema schema = openAPI.getComponents().getSchemas().get("AnyOfTest");
-        assertEquals(schema.getAnyOf().size(), 2);
+        assertEquals(schema.getAnyOf().size(), 4);
         assertNull(schema.getNullable());
 
         Schema schema2 = openAPI.getComponents().getSchemas().get("OneOfTest");
-        assertEquals(schema2.getOneOf().size(), 2);
+        assertEquals(schema2.getOneOf().size(), 4);
         assertNull(schema2.getNullable());
 
         Schema schema5 = openAPI.getComponents().getSchemas().get("OneOfNullableTest");
@@ -168,6 +168,7 @@ public class OpenAPINormalizerTest {
         Schema schema4 = openAPI.getComponents().getSchemas().get("OneOfTest");
         assertNull(schema4.getOneOf());
         assertTrue(schema4 instanceof IntegerSchema);
+        assertTrue(schema4.getNullable());
 
         Schema schema6 = openAPI.getComponents().getSchemas().get("OneOfNullableTest");
         assertEquals(schema6.getOneOf().size(), 2);
@@ -273,7 +274,7 @@ public class OpenAPINormalizerTest {
         OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_0/convertEnumNullToNullable_test.yaml");
 
         Schema schema = openAPI.getComponents().getSchemas().get("AnyOfTest");
-        assertEquals(schema.getAnyOf().size(), 3);
+        assertEquals(schema.getAnyOf().size(), 4);
         assertNull(schema.getNullable());
 
         Map<String, String> options = new HashMap<>();
@@ -292,7 +293,7 @@ public class OpenAPINormalizerTest {
         OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_0/convertEnumNullToNullable_test.yaml");
 
         Schema schema = openAPI.getComponents().getSchemas().get("AnyOfTest");
-        assertEquals(schema.getAnyOf().size(), 3);
+        assertEquals(schema.getAnyOf().size(), 4);
         assertNull(schema.getNullable());
 
         Map<String, String> options = new HashMap<>();
@@ -313,7 +314,7 @@ public class OpenAPINormalizerTest {
 
         // before test
         Schema schema = openAPI.getComponents().getSchemas().get("AnyOfTest");
-        assertEquals(schema.getAnyOf().size(), 3);
+        assertEquals(schema.getAnyOf().size(), 4);
         assertNull(schema.getNullable());
 
         Map<String, String> options = new HashMap<>();
@@ -323,7 +324,7 @@ public class OpenAPINormalizerTest {
 
         // checks should be the same after test
         Schema schema3 = openAPI.getComponents().getSchemas().get("AnyOfTest");
-        assertEquals(schema3.getAnyOf().size(), 3);
+        assertEquals(schema3.getAnyOf().size(), 4);
         assertNull(schema3.getNullable());
     }
 
diff --git a/modules/openapi-generator/src/test/resources/3_0/convertEnumNullToNullable_test.yaml b/modules/openapi-generator/src/test/resources/3_0/convertEnumNullToNullable_test.yaml
index ab97e13abbd..ad4bd77a6f3 100644
--- a/modules/openapi-generator/src/test/resources/3_0/convertEnumNullToNullable_test.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/convertEnumNullToNullable_test.yaml
@@ -31,12 +31,17 @@ components:
       anyOf:
         - type: string
         - $ref: '#/components/schemas/EnumString'
+        - $ref: '#/components/schemas/EnumNullString'
         - $ref: '#/components/schemas/EnumNull'
     EnumString:
       type: string
       enum:
         - A
         - B
+    EnumNullString:
+      type: string
+      enum:
+        - 'null'
     EnumNull:
       type: string
       enum:
diff --git a/modules/openapi-generator/src/test/resources/3_0/simplifyOneOfAnyOf_test.yaml b/modules/openapi-generator/src/test/resources/3_0/simplifyOneOfAnyOf_test.yaml
index 7130fa9ba9a..9b3ae6acd61 100644
--- a/modules/openapi-generator/src/test/resources/3_0/simplifyOneOfAnyOf_test.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/simplifyOneOfAnyOf_test.yaml
@@ -30,11 +30,15 @@ components:
       description: to test anyOf
       anyOf:
         - type: string
+        - type: 'null'
         - type: null
+        - $ref: null
     OneOfTest:
       description: to test oneOf
       oneOf:
         - type: integer
+        - type: 'null'
+        - type: null
         - $ref: null
     OneOfNullableTest:
       description: to test oneOf nullable
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Model/Drawing.cs
index f52bb31aab2..48b90884333 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-ConditionalSerialization/src/Org.OpenAPITools/Model/Drawing.cs
@@ -90,7 +90,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull
         {
             get{ return _ShapeOrNull;}
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/models/Drawing.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/models/Drawing.md
index fcee9662afb..215793515f6 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/models/Drawing.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/docs/models/Drawing.md
@@ -5,9 +5,9 @@
 Name | Type | Description | Notes
 ------------ | ------------- | ------------- | -------------
 **MainShape** | [**Shape**](Shape.md) |  | [optional] 
-**ShapeOrNull** | [**ShapeOrNull**](ShapeOrNull.md) |  | [optional] 
 **Shapes** | [**List&lt;Shape&gt;**](Shape.md) |  | [optional] 
 **NullableShape** | [**NullableShape**](NullableShape.md) |  | [optional] 
+**ShapeOrNull** | [**ShapeOrNull**](ShapeOrNull.md) |  | [optional] 
 
 [[Back to Model list]](../../README.md#documentation-for-models) [[Back to API list]](../../README.md#documentation-for-api-endpoints) [[Back to README]](../../README.md)
 
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Model/Drawing.cs
index 26ebf63a684..f21f1f207db 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Model/Drawing.cs
@@ -34,16 +34,16 @@ namespace Org.OpenAPITools.Model
         /// Initializes a new instance of the <see cref="Drawing" /> class.
         /// </summary>
         /// <param name="mainShape">mainShape</param>
-        /// <param name="shapeOrNull">shapeOrNull</param>
         /// <param name="shapes">shapes</param>
         /// <param name="nullableShape">nullableShape</param>
+        /// <param name="shapeOrNull">shapeOrNull</param>
         [JsonConstructor]
-        public Drawing(Shape mainShape, ShapeOrNull shapeOrNull, List<Shape> shapes, NullableShape? nullableShape = default) : base()
+        public Drawing(Shape mainShape, List<Shape> shapes, NullableShape? nullableShape = default, ShapeOrNull? shapeOrNull = default) : base()
         {
             MainShape = mainShape;
-            ShapeOrNull = shapeOrNull;
             Shapes = shapes;
             NullableShape = nullableShape;
+            ShapeOrNull = shapeOrNull;
             OnCreated();
         }
 
@@ -55,12 +55,6 @@ namespace Org.OpenAPITools.Model
         [JsonPropertyName("mainShape")]
         public Shape MainShape { get; set; }
 
-        /// <summary>
-        /// Gets or Sets ShapeOrNull
-        /// </summary>
-        [JsonPropertyName("shapeOrNull")]
-        public ShapeOrNull ShapeOrNull { get; set; }
-
         /// <summary>
         /// Gets or Sets Shapes
         /// </summary>
@@ -73,6 +67,12 @@ namespace Org.OpenAPITools.Model
         [JsonPropertyName("nullableShape")]
         public NullableShape? NullableShape { get; set; }
 
+        /// <summary>
+        /// Gets or Sets ShapeOrNull
+        /// </summary>
+        [JsonPropertyName("shapeOrNull")]
+        public ShapeOrNull? ShapeOrNull { get; set; }
+
         /// <summary>
         /// Returns the string presentation of the object
         /// </summary>
@@ -83,9 +83,9 @@ namespace Org.OpenAPITools.Model
             sb.Append("class Drawing {\n");
             sb.Append("  ").Append(base.ToString()?.Replace("\n", "\n  ")).Append("\n");
             sb.Append("  MainShape: ").Append(MainShape).Append("\n");
-            sb.Append("  ShapeOrNull: ").Append(ShapeOrNull).Append("\n");
             sb.Append("  Shapes: ").Append(Shapes).Append("\n");
             sb.Append("  NullableShape: ").Append(NullableShape).Append("\n");
+            sb.Append("  ShapeOrNull: ").Append(ShapeOrNull).Append("\n");
             sb.Append("}\n");
             return sb.ToString();
         }
@@ -124,9 +124,9 @@ namespace Org.OpenAPITools.Model
             JsonTokenType startingTokenType = utf8JsonReader.TokenType;
 
             Shape? mainShape = default;
-            ShapeOrNull? shapeOrNull = default;
             List<Shape>? shapes = default;
             NullableShape? nullableShape = default;
+            ShapeOrNull? shapeOrNull = default;
 
             while (utf8JsonReader.Read())
             {
@@ -147,10 +147,6 @@ namespace Org.OpenAPITools.Model
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 mainShape = JsonSerializer.Deserialize<Shape>(ref utf8JsonReader, jsonSerializerOptions);
                             break;
-                        case "shapeOrNull":
-                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
-                                shapeOrNull = JsonSerializer.Deserialize<ShapeOrNull>(ref utf8JsonReader, jsonSerializerOptions);
-                            break;
                         case "shapes":
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 shapes = JsonSerializer.Deserialize<List<Shape>>(ref utf8JsonReader, jsonSerializerOptions);
@@ -159,6 +155,10 @@ namespace Org.OpenAPITools.Model
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 nullableShape = JsonSerializer.Deserialize<NullableShape>(ref utf8JsonReader, jsonSerializerOptions);
                             break;
+                        case "shapeOrNull":
+                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
+                                shapeOrNull = JsonSerializer.Deserialize<ShapeOrNull>(ref utf8JsonReader, jsonSerializerOptions);
+                            break;
                         default:
                             break;
                     }
@@ -168,13 +168,10 @@ namespace Org.OpenAPITools.Model
             if (mainShape == null)
                 throw new ArgumentNullException(nameof(mainShape), "Property is required for class Drawing.");
 
-            if (shapeOrNull == null)
-                throw new ArgumentNullException(nameof(shapeOrNull), "Property is required for class Drawing.");
-
             if (shapes == null)
                 throw new ArgumentNullException(nameof(shapes), "Property is required for class Drawing.");
 
-            return new Drawing(mainShape, shapeOrNull, shapes, nullableShape);
+            return new Drawing(mainShape, shapes, nullableShape, shapeOrNull);
         }
 
         /// <summary>
@@ -190,12 +187,12 @@ namespace Org.OpenAPITools.Model
 
             writer.WritePropertyName("mainShape");
             JsonSerializer.Serialize(writer, drawing.MainShape, jsonSerializerOptions);
-            writer.WritePropertyName("shapeOrNull");
-            JsonSerializer.Serialize(writer, drawing.ShapeOrNull, jsonSerializerOptions);
             writer.WritePropertyName("shapes");
             JsonSerializer.Serialize(writer, drawing.Shapes, jsonSerializerOptions);
             writer.WritePropertyName("nullableShape");
             JsonSerializer.Serialize(writer, drawing.NullableShape, jsonSerializerOptions);
+            writer.WritePropertyName("shapeOrNull");
+            JsonSerializer.Serialize(writer, drawing.ShapeOrNull, jsonSerializerOptions);
 
             writer.WriteEndObject();
         }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/models/Drawing.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/models/Drawing.md
index fcee9662afb..215793515f6 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/models/Drawing.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/docs/models/Drawing.md
@@ -5,9 +5,9 @@
 Name | Type | Description | Notes
 ------------ | ------------- | ------------- | -------------
 **MainShape** | [**Shape**](Shape.md) |  | [optional] 
-**ShapeOrNull** | [**ShapeOrNull**](ShapeOrNull.md) |  | [optional] 
 **Shapes** | [**List&lt;Shape&gt;**](Shape.md) |  | [optional] 
 **NullableShape** | [**NullableShape**](NullableShape.md) |  | [optional] 
+**ShapeOrNull** | [**ShapeOrNull**](ShapeOrNull.md) |  | [optional] 
 
 [[Back to Model list]](../../README.md#documentation-for-models) [[Back to API list]](../../README.md#documentation-for-api-endpoints) [[Back to README]](../../README.md)
 
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Model/Drawing.cs
index 32f83dd034b..c1d26496fc7 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-net6.0/src/Org.OpenAPITools/Model/Drawing.cs
@@ -32,16 +32,16 @@ namespace Org.OpenAPITools.Model
         /// Initializes a new instance of the <see cref="Drawing" /> class.
         /// </summary>
         /// <param name="mainShape">mainShape</param>
-        /// <param name="shapeOrNull">shapeOrNull</param>
         /// <param name="shapes">shapes</param>
         /// <param name="nullableShape">nullableShape</param>
+        /// <param name="shapeOrNull">shapeOrNull</param>
         [JsonConstructor]
-        public Drawing(Shape mainShape, ShapeOrNull shapeOrNull, List<Shape> shapes, NullableShape nullableShape = default) : base()
+        public Drawing(Shape mainShape, List<Shape> shapes, NullableShape nullableShape = default, ShapeOrNull shapeOrNull = default) : base()
         {
             MainShape = mainShape;
-            ShapeOrNull = shapeOrNull;
             Shapes = shapes;
             NullableShape = nullableShape;
+            ShapeOrNull = shapeOrNull;
             OnCreated();
         }
 
@@ -53,12 +53,6 @@ namespace Org.OpenAPITools.Model
         [JsonPropertyName("mainShape")]
         public Shape MainShape { get; set; }
 
-        /// <summary>
-        /// Gets or Sets ShapeOrNull
-        /// </summary>
-        [JsonPropertyName("shapeOrNull")]
-        public ShapeOrNull ShapeOrNull { get; set; }
-
         /// <summary>
         /// Gets or Sets Shapes
         /// </summary>
@@ -71,6 +65,12 @@ namespace Org.OpenAPITools.Model
         [JsonPropertyName("nullableShape")]
         public NullableShape NullableShape { get; set; }
 
+        /// <summary>
+        /// Gets or Sets ShapeOrNull
+        /// </summary>
+        [JsonPropertyName("shapeOrNull")]
+        public ShapeOrNull ShapeOrNull { get; set; }
+
         /// <summary>
         /// Returns the string presentation of the object
         /// </summary>
@@ -81,9 +81,9 @@ namespace Org.OpenAPITools.Model
             sb.Append("class Drawing {\n");
             sb.Append("  ").Append(base.ToString()?.Replace("\n", "\n  ")).Append("\n");
             sb.Append("  MainShape: ").Append(MainShape).Append("\n");
-            sb.Append("  ShapeOrNull: ").Append(ShapeOrNull).Append("\n");
             sb.Append("  Shapes: ").Append(Shapes).Append("\n");
             sb.Append("  NullableShape: ").Append(NullableShape).Append("\n");
+            sb.Append("  ShapeOrNull: ").Append(ShapeOrNull).Append("\n");
             sb.Append("}\n");
             return sb.ToString();
         }
@@ -122,9 +122,9 @@ namespace Org.OpenAPITools.Model
             JsonTokenType startingTokenType = utf8JsonReader.TokenType;
 
             Shape mainShape = default;
-            ShapeOrNull shapeOrNull = default;
             List<Shape> shapes = default;
             NullableShape nullableShape = default;
+            ShapeOrNull shapeOrNull = default;
 
             while (utf8JsonReader.Read())
             {
@@ -145,10 +145,6 @@ namespace Org.OpenAPITools.Model
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 mainShape = JsonSerializer.Deserialize<Shape>(ref utf8JsonReader, jsonSerializerOptions);
                             break;
-                        case "shapeOrNull":
-                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
-                                shapeOrNull = JsonSerializer.Deserialize<ShapeOrNull>(ref utf8JsonReader, jsonSerializerOptions);
-                            break;
                         case "shapes":
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 shapes = JsonSerializer.Deserialize<List<Shape>>(ref utf8JsonReader, jsonSerializerOptions);
@@ -157,6 +153,10 @@ namespace Org.OpenAPITools.Model
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 nullableShape = JsonSerializer.Deserialize<NullableShape>(ref utf8JsonReader, jsonSerializerOptions);
                             break;
+                        case "shapeOrNull":
+                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
+                                shapeOrNull = JsonSerializer.Deserialize<ShapeOrNull>(ref utf8JsonReader, jsonSerializerOptions);
+                            break;
                         default:
                             break;
                     }
@@ -166,13 +166,10 @@ namespace Org.OpenAPITools.Model
             if (mainShape == null)
                 throw new ArgumentNullException(nameof(mainShape), "Property is required for class Drawing.");
 
-            if (shapeOrNull == null)
-                throw new ArgumentNullException(nameof(shapeOrNull), "Property is required for class Drawing.");
-
             if (shapes == null)
                 throw new ArgumentNullException(nameof(shapes), "Property is required for class Drawing.");
 
-            return new Drawing(mainShape, shapeOrNull, shapes, nullableShape);
+            return new Drawing(mainShape, shapes, nullableShape, shapeOrNull);
         }
 
         /// <summary>
@@ -188,12 +185,12 @@ namespace Org.OpenAPITools.Model
 
             writer.WritePropertyName("mainShape");
             JsonSerializer.Serialize(writer, drawing.MainShape, jsonSerializerOptions);
-            writer.WritePropertyName("shapeOrNull");
-            JsonSerializer.Serialize(writer, drawing.ShapeOrNull, jsonSerializerOptions);
             writer.WritePropertyName("shapes");
             JsonSerializer.Serialize(writer, drawing.Shapes, jsonSerializerOptions);
             writer.WritePropertyName("nullableShape");
             JsonSerializer.Serialize(writer, drawing.NullableShape, jsonSerializerOptions);
+            writer.WritePropertyName("shapeOrNull");
+            JsonSerializer.Serialize(writer, drawing.ShapeOrNull, jsonSerializerOptions);
 
             writer.WriteEndObject();
         }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/models/Drawing.md b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/models/Drawing.md
index fcee9662afb..215793515f6 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/models/Drawing.md
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/docs/models/Drawing.md
@@ -5,9 +5,9 @@
 Name | Type | Description | Notes
 ------------ | ------------- | ------------- | -------------
 **MainShape** | [**Shape**](Shape.md) |  | [optional] 
-**ShapeOrNull** | [**ShapeOrNull**](ShapeOrNull.md) |  | [optional] 
 **Shapes** | [**List&lt;Shape&gt;**](Shape.md) |  | [optional] 
 **NullableShape** | [**NullableShape**](NullableShape.md) |  | [optional] 
+**ShapeOrNull** | [**ShapeOrNull**](ShapeOrNull.md) |  | [optional] 
 
 [[Back to Model list]](../../README.md#documentation-for-models) [[Back to API list]](../../README.md#documentation-for-api-endpoints) [[Back to README]](../../README.md)
 
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Model/Drawing.cs
index 32f83dd034b..c1d26496fc7 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-generichost-netstandard2.0/src/Org.OpenAPITools/Model/Drawing.cs
@@ -32,16 +32,16 @@ namespace Org.OpenAPITools.Model
         /// Initializes a new instance of the <see cref="Drawing" /> class.
         /// </summary>
         /// <param name="mainShape">mainShape</param>
-        /// <param name="shapeOrNull">shapeOrNull</param>
         /// <param name="shapes">shapes</param>
         /// <param name="nullableShape">nullableShape</param>
+        /// <param name="shapeOrNull">shapeOrNull</param>
         [JsonConstructor]
-        public Drawing(Shape mainShape, ShapeOrNull shapeOrNull, List<Shape> shapes, NullableShape nullableShape = default) : base()
+        public Drawing(Shape mainShape, List<Shape> shapes, NullableShape nullableShape = default, ShapeOrNull shapeOrNull = default) : base()
         {
             MainShape = mainShape;
-            ShapeOrNull = shapeOrNull;
             Shapes = shapes;
             NullableShape = nullableShape;
+            ShapeOrNull = shapeOrNull;
             OnCreated();
         }
 
@@ -53,12 +53,6 @@ namespace Org.OpenAPITools.Model
         [JsonPropertyName("mainShape")]
         public Shape MainShape { get; set; }
 
-        /// <summary>
-        /// Gets or Sets ShapeOrNull
-        /// </summary>
-        [JsonPropertyName("shapeOrNull")]
-        public ShapeOrNull ShapeOrNull { get; set; }
-
         /// <summary>
         /// Gets or Sets Shapes
         /// </summary>
@@ -71,6 +65,12 @@ namespace Org.OpenAPITools.Model
         [JsonPropertyName("nullableShape")]
         public NullableShape NullableShape { get; set; }
 
+        /// <summary>
+        /// Gets or Sets ShapeOrNull
+        /// </summary>
+        [JsonPropertyName("shapeOrNull")]
+        public ShapeOrNull ShapeOrNull { get; set; }
+
         /// <summary>
         /// Returns the string presentation of the object
         /// </summary>
@@ -81,9 +81,9 @@ namespace Org.OpenAPITools.Model
             sb.Append("class Drawing {\n");
             sb.Append("  ").Append(base.ToString()?.Replace("\n", "\n  ")).Append("\n");
             sb.Append("  MainShape: ").Append(MainShape).Append("\n");
-            sb.Append("  ShapeOrNull: ").Append(ShapeOrNull).Append("\n");
             sb.Append("  Shapes: ").Append(Shapes).Append("\n");
             sb.Append("  NullableShape: ").Append(NullableShape).Append("\n");
+            sb.Append("  ShapeOrNull: ").Append(ShapeOrNull).Append("\n");
             sb.Append("}\n");
             return sb.ToString();
         }
@@ -122,9 +122,9 @@ namespace Org.OpenAPITools.Model
             JsonTokenType startingTokenType = utf8JsonReader.TokenType;
 
             Shape mainShape = default;
-            ShapeOrNull shapeOrNull = default;
             List<Shape> shapes = default;
             NullableShape nullableShape = default;
+            ShapeOrNull shapeOrNull = default;
 
             while (utf8JsonReader.Read())
             {
@@ -145,10 +145,6 @@ namespace Org.OpenAPITools.Model
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 mainShape = JsonSerializer.Deserialize<Shape>(ref utf8JsonReader, jsonSerializerOptions);
                             break;
-                        case "shapeOrNull":
-                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
-                                shapeOrNull = JsonSerializer.Deserialize<ShapeOrNull>(ref utf8JsonReader, jsonSerializerOptions);
-                            break;
                         case "shapes":
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 shapes = JsonSerializer.Deserialize<List<Shape>>(ref utf8JsonReader, jsonSerializerOptions);
@@ -157,6 +153,10 @@ namespace Org.OpenAPITools.Model
                             if (utf8JsonReader.TokenType != JsonTokenType.Null)
                                 nullableShape = JsonSerializer.Deserialize<NullableShape>(ref utf8JsonReader, jsonSerializerOptions);
                             break;
+                        case "shapeOrNull":
+                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
+                                shapeOrNull = JsonSerializer.Deserialize<ShapeOrNull>(ref utf8JsonReader, jsonSerializerOptions);
+                            break;
                         default:
                             break;
                     }
@@ -166,13 +166,10 @@ namespace Org.OpenAPITools.Model
             if (mainShape == null)
                 throw new ArgumentNullException(nameof(mainShape), "Property is required for class Drawing.");
 
-            if (shapeOrNull == null)
-                throw new ArgumentNullException(nameof(shapeOrNull), "Property is required for class Drawing.");
-
             if (shapes == null)
                 throw new ArgumentNullException(nameof(shapes), "Property is required for class Drawing.");
 
-            return new Drawing(mainShape, shapeOrNull, shapes, nullableShape);
+            return new Drawing(mainShape, shapes, nullableShape, shapeOrNull);
         }
 
         /// <summary>
@@ -188,12 +185,12 @@ namespace Org.OpenAPITools.Model
 
             writer.WritePropertyName("mainShape");
             JsonSerializer.Serialize(writer, drawing.MainShape, jsonSerializerOptions);
-            writer.WritePropertyName("shapeOrNull");
-            JsonSerializer.Serialize(writer, drawing.ShapeOrNull, jsonSerializerOptions);
             writer.WritePropertyName("shapes");
             JsonSerializer.Serialize(writer, drawing.Shapes, jsonSerializerOptions);
             writer.WritePropertyName("nullableShape");
             JsonSerializer.Serialize(writer, drawing.NullableShape, jsonSerializerOptions);
+            writer.WritePropertyName("shapeOrNull");
+            JsonSerializer.Serialize(writer, drawing.ShapeOrNull, jsonSerializerOptions);
 
             writer.WriteEndObject();
         }
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Model/Drawing.cs
index ae441f94c69..47ce8968ddd 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-httpclient/src/Org.OpenAPITools/Model/Drawing.cs
@@ -57,7 +57,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Model/Drawing.cs
index 0a759b37b3b..3663c769fc2 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net47/src/Org.OpenAPITools/Model/Drawing.cs
@@ -56,7 +56,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/src/Org.OpenAPITools/Model/Drawing.cs
index 0a759b37b3b..3663c769fc2 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net48/src/Org.OpenAPITools/Model/Drawing.cs
@@ -56,7 +56,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Model/Drawing.cs
index 0a759b37b3b..3663c769fc2 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-net5.0/src/Org.OpenAPITools/Model/Drawing.cs
@@ -56,7 +56,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/src/Org.OpenAPITools/Model/Drawing.cs
index 7ceaa0ce3b1..0d19d8d09ed 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient-unityWebRequest/src/Org.OpenAPITools/Model/Drawing.cs
@@ -54,7 +54,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClient/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Model/Drawing.cs
index 0a759b37b3b..3663c769fc2 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClient/src/Org.OpenAPITools/Model/Drawing.cs
@@ -56,7 +56,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/api/openapi.yaml b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/api/openapi.yaml
index 22e8da5234e..ba9171e6cbe 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/api/openapi.yaml
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/api/openapi.yaml
@@ -1971,8 +1971,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2021,8 +2021,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Model/Drawing.cs b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Model/Drawing.cs
index 0a759b37b3b..3663c769fc2 100644
--- a/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Model/Drawing.cs
+++ b/samples/client/petstore/csharp-netcore/OpenAPIClientCore/src/Org.OpenAPITools/Model/Drawing.cs
@@ -56,7 +56,7 @@ namespace Org.OpenAPITools.Model
         /// <summary>
         /// Gets or Sets ShapeOrNull
         /// </summary>
-        [DataMember(Name = "shapeOrNull", EmitDefaultValue = false)]
+        [DataMember(Name = "shapeOrNull", EmitDefaultValue = true)]
         public ShapeOrNull ShapeOrNull { get; set; }
 
         /// <summary>
diff --git a/samples/client/petstore/java/jersey3/api/openapi.yaml b/samples/client/petstore/java/jersey3/api/openapi.yaml
index 3e2a497d7a8..b02c0b26cb7 100644
--- a/samples/client/petstore/java/jersey3/api/openapi.yaml
+++ b/samples/client/petstore/java/jersey3/api/openapi.yaml
@@ -1973,8 +1973,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2023,8 +2023,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/java/jersey3/src/main/java/org/openapitools/client/model/Drawing.java b/samples/client/petstore/java/jersey3/src/main/java/org/openapitools/client/model/Drawing.java
index d9dd5f94e52..55534b1e127 100644
--- a/samples/client/petstore/java/jersey3/src/main/java/org/openapitools/client/model/Drawing.java
+++ b/samples/client/petstore/java/jersey3/src/main/java/org/openapitools/client/model/Drawing.java
@@ -55,7 +55,7 @@ public class Drawing {
   private Shape mainShape;
 
   public static final String JSON_PROPERTY_SHAPE_OR_NULL = "shapeOrNull";
-  private ShapeOrNull shapeOrNull;
+  private JsonNullable<ShapeOrNull> shapeOrNull = JsonNullable.<ShapeOrNull>undefined();
 
   public static final String JSON_PROPERTY_NULLABLE_SHAPE = "nullableShape";
   private JsonNullable<NullableShape> nullableShape = JsonNullable.<NullableShape>undefined();
@@ -92,7 +92,7 @@ public class Drawing {
 
 
   public Drawing shapeOrNull(ShapeOrNull shapeOrNull) {
-    this.shapeOrNull = shapeOrNull;
+    this.shapeOrNull = JsonNullable.<ShapeOrNull>of(shapeOrNull);
     return this;
   }
 
@@ -101,20 +101,28 @@ public class Drawing {
    * @return shapeOrNull
   **/
   @jakarta.annotation.Nullable
-  @JsonProperty(JSON_PROPERTY_SHAPE_OR_NULL)
-  @JsonInclude(value = JsonInclude.Include.USE_DEFAULTS)
+  @JsonIgnore
 
   public ShapeOrNull getShapeOrNull() {
-    return shapeOrNull;
+        return shapeOrNull.orElse(null);
   }
 
-
   @JsonProperty(JSON_PROPERTY_SHAPE_OR_NULL)
   @JsonInclude(value = JsonInclude.Include.USE_DEFAULTS)
-  public void setShapeOrNull(ShapeOrNull shapeOrNull) {
+
+  public JsonNullable<ShapeOrNull> getShapeOrNull_JsonNullable() {
+    return shapeOrNull;
+  }
+  
+  @JsonProperty(JSON_PROPERTY_SHAPE_OR_NULL)
+  public void setShapeOrNull_JsonNullable(JsonNullable<ShapeOrNull> shapeOrNull) {
     this.shapeOrNull = shapeOrNull;
   }
 
+  public void setShapeOrNull(ShapeOrNull shapeOrNull) {
+    this.shapeOrNull = JsonNullable.<ShapeOrNull>of(shapeOrNull);
+  }
+
 
   public Drawing nullableShape(NullableShape nullableShape) {
     this.nullableShape = JsonNullable.<NullableShape>of(nullableShape);
@@ -232,7 +240,7 @@ public class Drawing {
     }
     Drawing drawing = (Drawing) o;
     return Objects.equals(this.mainShape, drawing.mainShape) &&
-        Objects.equals(this.shapeOrNull, drawing.shapeOrNull) &&
+        equalsNullable(this.shapeOrNull, drawing.shapeOrNull) &&
         equalsNullable(this.nullableShape, drawing.nullableShape) &&
         Objects.equals(this.shapes, drawing.shapes)&&
         Objects.equals(this.additionalProperties, drawing.additionalProperties);
@@ -244,7 +252,7 @@ public class Drawing {
 
   @Override
   public int hashCode() {
-    return Objects.hash(mainShape, shapeOrNull, hashCodeNullable(nullableShape), shapes, additionalProperties);
+    return Objects.hash(mainShape, hashCodeNullable(shapeOrNull), hashCodeNullable(nullableShape), shapes, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {
diff --git a/samples/client/petstore/java/okhttp-gson/api/openapi.yaml b/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
index 954401cb158..a879895762c 100644
--- a/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
+++ b/samples/client/petstore/java/okhttp-gson/api/openapi.yaml
@@ -1973,8 +1973,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2023,8 +2023,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/client/petstore/typescript-axios/builds/test-petstore/api.ts b/samples/client/petstore/typescript-axios/builds/test-petstore/api.ts
index c0951271f3d..647afdc57dc 100644
--- a/samples/client/petstore/typescript-axios/builds/test-petstore/api.ts
+++ b/samples/client/petstore/typescript-axios/builds/test-petstore/api.ts
@@ -512,7 +512,7 @@ export interface Drawing {
      * @type {ShapeOrNull}
      * @memberof Drawing
      */
-    'shapeOrNull'?: ShapeOrNull;
+    'shapeOrNull'?: ShapeOrNull | null;
     /**
      * 
      * @type {NullableShape}
@@ -846,7 +846,7 @@ export type Fruit = Apple | Banana;
  * @type FruitReq
  * @export
  */
-export type FruitReq = AppleReq | BananaReq | Null;
+export type FruitReq = AppleReq | BananaReq;
 
 /**
  * 
@@ -1520,7 +1520,7 @@ export interface ShapeInterface {
  * The value may be a shape or the \'null\' value. This is introduced in OAS schema >= 3.1.
  * @export
  */
-export type ShapeOrNull = Null | Quadrilateral | Triangle;
+export type ShapeOrNull = Quadrilateral | Triangle;
 
 /**
  * 
diff --git a/samples/openapi3/client/petstore/java/jersey2-java8/api/openapi.yaml b/samples/openapi3/client/petstore/java/jersey2-java8/api/openapi.yaml
index 3e2a497d7a8..b02c0b26cb7 100644
--- a/samples/openapi3/client/petstore/java/jersey2-java8/api/openapi.yaml
+++ b/samples/openapi3/client/petstore/java/jersey2-java8/api/openapi.yaml
@@ -1973,8 +1973,8 @@ components:
           type: string
     fruitReq:
       additionalProperties: false
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/appleReq'
       - $ref: '#/components/schemas/bananaReq'
     appleReq:
@@ -2023,8 +2023,8 @@ components:
         in OAS schema >= 3.1.
       discriminator:
         propertyName: shapeType
+      nullable: true
       oneOf:
-      - type: "null"
       - $ref: '#/components/schemas/Triangle'
       - $ref: '#/components/schemas/Quadrilateral'
     NullableShape:
diff --git a/samples/openapi3/client/petstore/java/jersey2-java8/src/main/java/org/openapitools/client/model/Drawing.java b/samples/openapi3/client/petstore/java/jersey2-java8/src/main/java/org/openapitools/client/model/Drawing.java
index ac0adc5d30f..aecf20e2fbb 100644
--- a/samples/openapi3/client/petstore/java/jersey2-java8/src/main/java/org/openapitools/client/model/Drawing.java
+++ b/samples/openapi3/client/petstore/java/jersey2-java8/src/main/java/org/openapitools/client/model/Drawing.java
@@ -55,7 +55,7 @@ public class Drawing {
   private Shape mainShape;
 
   public static final String JSON_PROPERTY_SHAPE_OR_NULL = "shapeOrNull";
-  private ShapeOrNull shapeOrNull;
+  private JsonNullable<ShapeOrNull> shapeOrNull = JsonNullable.<ShapeOrNull>undefined();
 
   public static final String JSON_PROPERTY_NULLABLE_SHAPE = "nullableShape";
   private JsonNullable<NullableShape> nullableShape = JsonNullable.<NullableShape>undefined();
@@ -92,7 +92,7 @@ public class Drawing {
 
 
   public Drawing shapeOrNull(ShapeOrNull shapeOrNull) {
-    this.shapeOrNull = shapeOrNull;
+    this.shapeOrNull = JsonNullable.<ShapeOrNull>of(shapeOrNull);
     return this;
   }
 
@@ -101,20 +101,28 @@ public class Drawing {
    * @return shapeOrNull
   **/
   @javax.annotation.Nullable
-  @JsonProperty(JSON_PROPERTY_SHAPE_OR_NULL)
-  @JsonInclude(value = JsonInclude.Include.USE_DEFAULTS)
+  @JsonIgnore
 
   public ShapeOrNull getShapeOrNull() {
-    return shapeOrNull;
+        return shapeOrNull.orElse(null);
   }
 
-
   @JsonProperty(JSON_PROPERTY_SHAPE_OR_NULL)
   @JsonInclude(value = JsonInclude.Include.USE_DEFAULTS)
-  public void setShapeOrNull(ShapeOrNull shapeOrNull) {
+
+  public JsonNullable<ShapeOrNull> getShapeOrNull_JsonNullable() {
+    return shapeOrNull;
+  }
+  
+  @JsonProperty(JSON_PROPERTY_SHAPE_OR_NULL)
+  public void setShapeOrNull_JsonNullable(JsonNullable<ShapeOrNull> shapeOrNull) {
     this.shapeOrNull = shapeOrNull;
   }
 
+  public void setShapeOrNull(ShapeOrNull shapeOrNull) {
+    this.shapeOrNull = JsonNullable.<ShapeOrNull>of(shapeOrNull);
+  }
+
 
   public Drawing nullableShape(NullableShape nullableShape) {
     this.nullableShape = JsonNullable.<NullableShape>of(nullableShape);
@@ -232,7 +240,7 @@ public class Drawing {
     }
     Drawing drawing = (Drawing) o;
     return Objects.equals(this.mainShape, drawing.mainShape) &&
-        Objects.equals(this.shapeOrNull, drawing.shapeOrNull) &&
+        equalsNullable(this.shapeOrNull, drawing.shapeOrNull) &&
         equalsNullable(this.nullableShape, drawing.nullableShape) &&
         Objects.equals(this.shapes, drawing.shapes)&&
         Objects.equals(this.additionalProperties, drawing.additionalProperties);
@@ -244,7 +252,7 @@ public class Drawing {
 
   @Override
   public int hashCode() {
-    return Objects.hash(mainShape, shapeOrNull, hashCodeNullable(nullableShape), shapes, additionalProperties);
+    return Objects.hash(mainShape, hashCodeNullable(shapeOrNull), hashCodeNullable(nullableShape), shapes, additionalProperties);
   }
 
   private static <T> int hashCodeNullable(JsonNullable<T> a) {