Example of using `allOf`, `anyOf`, `oneOf`, and `not` to describe a schema in an OpenAPI spec. Also includes a nullable schema.

NewsArticle.swagger.yaml

# Invalid! Used for testing 3.0 version-guarding in the UI rendering.

swagger: "2.0"
servers: []
info:
  version: 1.0.0
  title: minimal
  description: News Articles ftw
paths:
  /users:
    get:
      responses:
        '200':
          description: hello world
definitions:
  NewsArticle:
    title: NewsArticle
    type: object
    nullable: true
    properties:
      id:
        type: integer
        format: int32
        x-mysql-type: int(11)
  allOfNewsArticle:
    title: allOfNewsArticle
    type: object
    allOf:
      - $ref: '#/definitions/NewsArticle'
      - properties:
          articleBody:
            type: string
  anyOfNewsArticle:
    title: anyOfNewsArticle
    type: object
    anyOf:
      - $ref: '#/definitions/NewsArticle'
      - properties:
          articleBody:
            type: string
  oneOfNewsArticle:
    title: oneOfNewsArticle
    type: object
    oneOf:
      - $ref: '#/definitions/NewsArticle'
      - properties:
          articleBody:
            type: string
  notNewsArticle:
    title: notNewsArticle
    type: object
    allOf:
      - $ref: '#/definitions/NewsArticle'
      - not:
          articleBody:
            type: string
  responses: {}
  parameters: {}
  examples: {}
  requestBodies: {}
  securitySchemes: {}
  headers: {}

NewsArticle.yaml

openapi: 3.0.0-RC1
servers: []
info:
  version: 1.0.0
  title: minimal
  description: News Articles ftw
paths:
  /users:
    get:
      responses:
        '200':
          description: hello world
components:
  schemas:
    NewsArticle:
      title: NewsArticle
      type: object
      nullable: true
      properties:
        id:
          type: integer
          format: int32
          x-mysql-type: int(11)
    allOfNewsArticle:
      title: allOfNewsArticle
      type: object
      allOf:
        - $ref: '#/components/schemas/NewsArticle'
        - properties:
            articleBody:
              type: string
    anyOfNewsArticle:
      title: anyOfNewsArticle
      type: object
      anyOf:
        - $ref: '#/components/schemas/NewsArticle'
        - properties:
            articleBody:
              type: string
    oneOfNewsArticle:
      title: oneOfNewsArticle
      type: object
      oneOf:
        - $ref: '#/components/schemas/NewsArticle'
        - properties:
            articleBody:
              type: string
    notNewsArticle:
      title: notNewsArticle
      type: object
      allOf:
        - $ref: '#/components/schemas/NewsArticle'
        - not:
            articleBody:
              type: string
  responses: {}
  parameters: {}
  examples: {}
  requestBodies: {}
  securitySchemes: {}
  headers: {}

The value of not should be a schema, so it should be

        - not:
            properties:
              articleBody:
                type: string

or

        - not:
            type: string

etc.