The official website for OAuth 2.0 is https://oauth.net/2/.
https://auth0.com/docs/protocols/oauth2 provides more information about the OAuth 2 protocol.
The OAuth 2.0 standard, RFC6749, is at https://tools.ietf.org/html/rfc6749.
Google provides comprehensive documentation and examples of OAuth 2.0 at https://developers.google.com/identity/protocols/OAuth2.