Yep, I ran into a few issues wiring up OAuth over the last few weeks building a ChatGPT connector so built this library to help folks get up and running quickly.

https://github.com/mcpauth/mcpauth/

Let me know if you have any questions on it

I came across https://github.com/systempromptio/systemprompt-mcp-server which has an OAuth 2.1 demo