gistfile1.txt

openapi: 3.0.0
info:
  title: Test API
  version: 0.0.1
servers:
  - url: 'localhost:3000/api'
    description: Local API URL.
paths:
  /test:
    post:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - $ref: '#/components/schemas/a'
                - $ref: '#/components/schemas/b'
      responses:
        '200':
          description: OK
components:
  schemas:
    a:
      type: integer
    b:
      type: string