diff --git a/packages/openapi-v3/src/__tests__/unit/x-ts-type.unit.ts b/packages/openapi-v3/src/__tests__/unit/x-ts-type.unit.ts
new file mode 100644
index 000000000000..7f6bd82a380d
--- /dev/null
+++ b/packages/openapi-v3/src/__tests__/unit/x-ts-type.unit.ts
@@ -0,0 +1,257 @@
+// Copyright IBM Corp. 2019. All Rights Reserved.
+// Node module: @loopback/openapi-v3
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+import {Model, model, property} from '@loopback/repository';
+import {expect} from '@loopback/testlab';
+import {RequestBodyObject, ResponseObject} from 'openapi3-ts';
+import {getControllerSpec} from '../..';
+import {get, post, requestBody} from '../../decorators';
+
+describe('x-ts-type is converted in the right places', () => {
+  // setup the models for use
+  @model()
+  class TestRequest extends Model {
+    @property({default: 1})
+    value: number;
+  }
+  @model()
+  class SuccessModel extends Model {
+    constructor(err: Partial<SuccessModel>) {
+      super(err);
+    }
+    @property({default: 'ok'})
+    message: string;
+  }
+
+  @model()
+  class FooError extends Model {
+    constructor(err: Partial<FooError>) {
+      super(err);
+    }
+    @property({default: 'foo'})
+    foo: string;
+  }
+
+  @model()
+  class NotError extends Model {
+    constructor(err: Partial<NotError>) {
+      super(err);
+    }
+    @property({default: true})
+    fail: boolean;
+  }
+
+  @model()
+  class BarError extends Model {
+    constructor(err: Partial<BarError>) {
+      super(err);
+    }
+    @property({default: 'bar'})
+    bar: string;
+  }
+
+  const testRequestSchema: RequestBodyObject = {
+    content: {
+      'application/json': {
+        schema: {
+          $ref: '#/components/schemas/TestRequest',
+        },
+      },
+    },
+  };
+  const successSchema: ResponseObject = {
+    description: 'Success',
+    content: {
+      'application/json': {
+        schema: {
+          $ref: '#/components/schemas/SuccessModel',
+        },
+      },
+    },
+  };
+
+  const notSchema: ResponseObject = {
+    description: 'Failure',
+    content: {
+      'application/json': {
+        schema: {
+          not: {$ref: '#/components/schemas/BarError'},
+        },
+      },
+    },
+  };
+  const fooBarSchema = (k: 'anyOf' | 'allOf' | 'oneOf'): ResponseObject => ({
+    description: 'Failure',
+    content: {
+      'application/json': {
+        schema: {
+          [k]: [
+            {$ref: '#/components/schemas/FooError'},
+            {$ref: '#/components/schemas/BarError'},
+          ],
+          not: {$ref: '#/components/schemas/NotError'},
+        },
+      },
+    },
+  });
+
+  it('Allows a simple request schema', () => {
+    class MyController {
+      @post('/greet')
+      greet(@requestBody() body: TestRequest) {
+        return 'Hello world!';
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].post.requestBody).to.eql(
+      testRequestSchema,
+    );
+  });
+
+  it('Does not process existing $ref responses', () => {
+    const successContent = {$ref: '#/components/schema/SomeReference'};
+    class MyController {
+      @post('/greet', {
+        responses: {
+          201: successContent,
+        },
+      })
+      greet(@requestBody() body: TestRequest) {
+        return 'Hello world!';
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].post.responses[201]).to.eql(
+      successContent,
+    );
+  });
+
+  it('Allows for a response schema using the spec', () => {
+    class MyController {
+      @get('/greet', {
+        responses: {
+          200: {
+            description: 'Success',
+            content: {
+              'application/json': {
+                schema: {
+                  'x-ts-type': SuccessModel,
+                },
+              },
+            },
+          },
+        },
+      })
+      greet() {
+        return new SuccessModel({message: 'hello, world'});
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].get.responses[200]).to.eql(successSchema);
+    expect(actualSpec.components?.schemas?.SuccessModel).to.not.be.undefined();
+  });
+
+  it('Allows `anyOf` responses', () => {
+    class MyController {
+      @get('/greet', {
+        responses: {
+          404: {
+            description: 'Failure',
+            content: {
+              'application/json': {
+                schema: {
+                  anyOf: [{'x-ts-type': FooError}, {'x-ts-type': BarError}],
+                  not: {'x-ts-type': NotError},
+                },
+              },
+            },
+          },
+        },
+      })
+      greet() {
+        throw new FooError({foo: 'foo'});
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].get.responses[404]).to.eql(
+      fooBarSchema('anyOf'),
+    );
+  });
+  it('Allows `allOf` responses', () => {
+    class MyController {
+      @get('/greet', {
+        responses: {
+          404: {
+            description: 'Failure',
+            content: {
+              'application/json': {
+                schema: {
+                  allOf: [{'x-ts-type': FooError}, {'x-ts-type': BarError}],
+                  not: {'x-ts-type': NotError},
+                },
+              },
+            },
+          },
+        },
+      })
+      greet() {
+        throw new FooError({foo: 'foo'});
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].get.responses[404]).to.eql(
+      fooBarSchema('allOf'),
+    );
+  });
+
+  it('Allows `oneOf` responses', () => {
+    class MyController {
+      @get('/greet', {
+        responses: {
+          404: {
+            description: 'Failure',
+            content: {
+              'application/json': {
+                schema: {
+                  oneOf: [{'x-ts-type': FooError}, {'x-ts-type': BarError}],
+                  not: {'x-ts-type': NotError},
+                },
+              },
+            },
+          },
+        },
+      })
+      greet() {
+        throw new FooError({foo: 'foo'});
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].get.responses[404]).to.eql(
+      fooBarSchema('oneOf'),
+    );
+  });
+  it('Allows `not` responses', () => {
+    class MyController {
+      @get('/greet', {
+        responses: {
+          404: {
+            description: 'Failure',
+            content: {
+              'application/json': {
+                schema: {
+                  not: {'x-ts-type': BarError},
+                },
+              },
+            },
+          },
+        },
+      })
+      greet() {
+        throw new FooError({foo: 'foo'});
+      }
+    }
+    const actualSpec = getControllerSpec(MyController);
+    expect(actualSpec.paths['/greet'].get.responses[404]).to.eql(notSchema);
+  });
+});
diff --git a/packages/openapi-v3/src/controller-spec.ts b/packages/openapi-v3/src/controller-spec.ts
index 41249c151291..a200e2a99da2 100644
--- a/packages/openapi-v3/src/controller-spec.ts
+++ b/packages/openapi-v3/src/controller-spec.ts
@@ -178,9 +178,11 @@ function resolveControllerSpec(constructor: Function): ControllerSpec {
 
       requestBody = requestBodies[0];
       debug('  requestBody for method %s: %j', op, requestBody);
+      /* istanbul ignore else */
       if (requestBody) {
         operationSpec.requestBody = requestBody;
 
+        /* istanbul ignore else */
         const content = requestBody.content || {};
         for (const mediaType in content) {
           processSchemaExtensions(spec, content[mediaType].schema);
@@ -233,6 +235,9 @@ function resolveControllerSpec(constructor: Function): ControllerSpec {
   return spec;
 }
 
+declare type MixKey = 'allOf' | 'anyOf' | 'oneOf';
+const SCHEMA_ARR_KEYS: MixKey[] = ['allOf', 'anyOf', 'oneOf'];
+
 /**
  * Resolve the x-ts-type in the schema object
  * @param spec - Controller spec
@@ -248,24 +253,51 @@ function processSchemaExtensions(
   assignRelatedSchemas(spec, schema.definitions);
   delete schema.definitions;
 
-  if (isReferenceObject(schema)) return;
+  /**
+   * check if we have been provided a `not`
+   * `not` is valid in many cases- here we're checking for
+   * `not: { schema: {'x-ts-type': SomeModel }}
+   */
+  if (schema.not) {
+    processSchemaExtensions(spec, schema.not);
+  }
+
+  /**
+   *  check for schema.allOf, schema.oneOf, schema.anyOf arrays first.
+   *  You cannot provide BOTH a defnintion AND one of these keywords.
+   */
+  /* istanbul ignore else */
+  const hasOwn = (prop: string) => schema?.hasOwnProperty(prop);
+
+  if (SCHEMA_ARR_KEYS.some(k => hasOwn(k))) {
+    SCHEMA_ARR_KEYS.forEach((k: MixKey) => {
+      /* istanbul ignore else */
+      if (schema?.[k] && Array.isArray(schema[k])) {
+        schema[k].forEach((r: (SchemaObject | ReferenceObject)[]) => {
+          processSchemaExtensions(spec, r);
+        });
+      }
+    });
+  } else {
+    if (isReferenceObject(schema)) return;
 
-  const tsType = schema[TS_TYPE_KEY];
-  debug('  %s => %o', TS_TYPE_KEY, tsType);
-  if (tsType) {
-    schema = resolveSchema(tsType, schema);
-    if (schema.$ref) generateOpenAPISchema(spec, tsType);
+    const tsType = schema[TS_TYPE_KEY];
+    debug('  %s => %o', TS_TYPE_KEY, tsType);
+    if (tsType) {
+      schema = resolveSchema(tsType, schema);
+      if (schema.$ref) generateOpenAPISchema(spec, tsType);
 
-    // We don't want a Function type in the final spec.
-    delete schema[TS_TYPE_KEY];
-    return;
-  }
-  if (schema.type === 'array') {
-    processSchemaExtensions(spec, schema.items);
-  } else if (schema.type === 'object') {
-    if (schema.properties) {
-      for (const p in schema.properties) {
-        processSchemaExtensions(spec, schema.properties[p]);
+      // We don't want a Function type in the final spec.
+      delete schema[TS_TYPE_KEY];
+      return;
+    }
+    if (schema.type === 'array') {
+      processSchemaExtensions(spec, schema.items);
+    } else if (schema.type === 'object') {
+      if (schema.properties) {
+        for (const p in schema.properties) {
+          processSchemaExtensions(spec, schema.properties[p]);
+        }
       }
     }
   }