I'm really trying to understand the difference between OpenID and OAuth? Maybe they're two totally separate things?
OpenID is about authentication (i.e. proving who you are), OAuth is about authorisation (i.e. to grant access to functionality/data/etc.. without having to deal with the original authentication).
OAuth could be used in external partner sites to allow access to protected data without them having to re-authenticate a user.
The blog post "OpenID versus OAuth from the user’s perspective" has a simple comparison of the two from the user's perspective and "OAuth-OpenID: You’re Barking Up the Wrong Tree if you Think They’re the Same Thing" has more information about it.