openapi.yml

---
openapi: 3.0.3
info:
  title: Example
  version: 1.0.0
servers:
  - url: 'https://example.com/'

paths:
  /callback:
    post:
      responses:
        '200':
          description: test
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Event"

components:
  schemas:
    Event:
      type: object
      oneOf:
        - $ref: "#/components/schemas/MyGreatEvent"
      discriminator:
        propertyName: type
        mapping:
          great: '#/components/schemas/MyGreatEvent'

    MyGreatEvent:
      allOf:
        - $ref: '#/components/schemas/Event'
        - type: object
          required:
            - type