This is not OAuth, in particular, what you need (OAuth is designed to authorize access for one website to specific private content stored on another), but OpenID - which is for authentication, not authorization. (Some sites, such as Twitter, provide authentication services through OAuth, but this is not what it is primarily for.) I used python-openid , which is pretty simple, or you can look at django-openid - although it assumes that it incomplete, you can get an idea of how to implement OpenID support.
The problem is a bit related to recognizing a copy and paste solution, but it is not particularly difficult to do.
Update: The piquadrat link (in the comments) is definitely worth it.