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 b22c33a73f1..83277ace932 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
@@ -852,8 +852,14 @@ public class OpenAPINormalizer {
             }
         }
 
-        if (!(schema instanceof JsonSchema) && (schema.getType() == null || schema.getType().equals("null")) && schema.get$ref() == null) {
-            return true;
+        if (schema instanceof JsonSchema) { // 3.1 spec
+            if (Boolean.TRUE.equals(schema.getNullable())) {
+                return true;
+            }
+        } else { // 3.0.x or 2.x spec
+            if ((schema.getType() == null || schema.getType().equals("null")) && schema.get$ref() == null) {
+                return true;
+            }
         }
 
         // convert referenced enum of null only to `nullable:true`
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 9783e9803b1..ee3dae2b4b3 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
@@ -513,4 +513,23 @@ public class OpenAPINormalizerTest {
         assertEquals(((Schema) schema4.getProperties().get("set_property")).getNullable(), true);
         assertEquals(((Schema) schema4.getProperties().get("map_property")).getNullable(), null);
     }
+
+    @Test
+    public void testOpenAPINormalizerSimplifyOneOfAnyOf31Spec() {
+        // to test the rule SIMPLIFY_ONEOF_ANYOF in 3.1 spec
+        OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_1/issue_18184.yaml");
+        // test spec contains anyOf with a ref to enum and another scheme type is null
+
+        Schema schema = openAPI.getComponents().getSchemas().get("Item");
+        assertEquals(((Schema) schema.getProperties().get("my_enum")).getAnyOf().size(), 2);
+
+        Map<String, String> options = new HashMap<>();
+        options.put("SIMPLIFY_ANYOF_STRING_AND_ENUM_STRING", "true");
+        OpenAPINormalizer openAPINormalizer = new OpenAPINormalizer(openAPI, options);
+        openAPINormalizer.normalize();
+
+        Schema schema2 = openAPI.getComponents().getSchemas().get("Item");
+        assertEquals(((Schema) schema2.getProperties().get("my_enum")).getAnyOf(), null);
+        assertEquals(((Schema) schema2.getProperties().get("my_enum")).get$ref(), "#/components/schemas/MyEnum");
+    }
 }
diff --git a/modules/openapi-generator/src/test/resources/3_1/issue_18184.yaml b/modules/openapi-generator/src/test/resources/3_1/issue_18184.yaml
new file mode 100644
index 00000000000..8c2e5c8e1ad
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_1/issue_18184.yaml
@@ -0,0 +1,77 @@
+---
+openapi: 3.1.0
+info:
+  title: simplify-oneof-anyof bug repro
+  version: 0.1.0
+paths:
+  "/items/{item_id}":
+    put:
+      operationId: update_item
+      summary: Update Item
+      parameters:
+      - in: path
+        name: item_id
+        required: true
+        schema:
+          title: Item Id
+          type: integer
+      - in: query
+        name: q
+        required: false
+        schema:
+          title: Q
+          anyOf:
+          - type: string
+          - type: 'null'
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              "$ref": "#/components/schemas/Item"
+      responses:
+        '200':
+          description: Successful Response
+          content:
+            application/json:
+              schema:
+                "$ref": "#/components/schemas/UpdateResponse"
+components:
+  schemas:
+    Item:
+      title: Item
+      type: object
+      properties:
+        is_offer:
+          title: Is Offer
+          anyOf:
+          - type: boolean
+          - type: 'null'
+        my_enum:
+          anyOf:
+          - "$ref": "#/components/schemas/MyEnum"
+          - type: 'null'
+        name:
+          title: Name
+          type: string
+      required:
+      - name
+    MyEnum:
+      title: MyEnum
+      type: string
+      enum:
+      - value-1
+      - value-2
+    UpdateResponse:
+      properties:
+        id:
+          title: Item Id
+          type: integer
+        name:
+          title: Item Name
+          type: string
+      required:
+      - name
+      - id
+      title: UpdateResponse
+      type: object