OSIN is a Go library implementing an OAuth 2.0 authorization server as defined in RFC 6749, with support for PKCE (RFC 7636). It provides a modular server with configurable storage backends, token generators, and client authentication. The library handles all standard grant types: authorization code, implicit, resource owner password credentials, client credentials, refresh token, and assertion. It also includes non-standard info endpoints. The architecture is centered around a `Server` struct with pluggable `Storage` and token generator interfaces. Key entry points are `HandleAuthorizeRequest`, `HandleAccessRequest`, and `HandleInfoRequest`, which users integrate into their HTTP handlers. The code is well-structured and tested, but the project is in maintenance-only mode with outdated dependencies (e.g., jwt-go v3, jose.v1) and no recent commits. It remains a solid reference implementation for OAuth2 server logic in Go.