@markcarey I created an OAuth2 endpoint on my website from scratch and made NodeBB utilize it successfully. It was the first time I engaged in such an endeavour and it wasn't easy!
I followed the examples provided in the OAuth2orize plugin's github.
Besically all you have to do is swap out the local db they use and implement your db logic. Also use some sort of UID generator for client ids/secrets.
If you have that done and are able to successfully authorize with some tool, for example Postman or the Google OAuth Playground it should be pretty straightforward from there to setup the NodeBB SSO OAuth plugin to fit your website.