spec.yaml

gistfile1.txt

openapi: 3.0.0
info:
  title: API
  version: 1.0.0
servers:
  - url: https://example.com
paths:
  /foo:
    post:
      requestBody:
        content:
          application/json:
            schema:
              type: object
              oneOf:
                - allOf:
                    - type: object
                      properties:
                        foo_id:
                          readOnly: true
                      required:
                        - foo_id
                - allOf:
                    - type: object
                      properties:
                        bar_id:
                          readOnly: true
                      required:
                        - bar_id
      responses:
        "200":
          description: "OK"