Nat Sakimura has written a valuable post describing how to write an OpenID Connect server in three simple steps. It shows by example how simple it is for OAuth servers to add OpenID Connect functionality. This post is a companion to his previous post OpenID Connect in a Nutshell, which described how simple it is to build OpenID Connect clients. If you’re involved in OpenID Connect in any way, or are considering becoming involved, these posts are well worth reading.
OpenID Connect Server in a Nutshell