I have seen posts that insist that OAuth is an orthogonally different thing from OpenID, because OpenID is about authenticating users, while OAuth is about giving access to certain services to a third party.
However, I know a lot of people uses OAuth as authentication anyway, as illustrated here, Wikipedia calls it "pseudo-authentication", but it looks like a valid way to go. Is there any drawback compared to using OpenID? if OAuth is OK for authentication, does it mean that OAuth can do everything OpenID does, but OpenID can't do somethign that OAuth does (like accessing services). Why would people use OpenID as opposed to OAuth then?
Note: I am a beginner, so please excuse me and point out if I miss something fundamental here.