It turns out OpenAI have a GitHub repository with a 26,000 line YAML OpenAPI schema describing their full API - the same API that's emulated by all sorts of other tools within the LLM ecosystem. Made some notes on that here, plus a YAML exploration tool:
https://simonwillison.net/2024/Dec/22/openai-openapi/
@simon you could also load it into https://editor.swagger.io