You probably need to add some info to your question regarding how you're doing your authentication. OAuth is an authorisation protocol, with a side-effect of authentication in some circumstances. So (for example) in Google OAuth, the user signs in to Google, not your website. Your website can test if the user is signed in to Google, and if so, obtain an access/id token to know who the user is.
So it all depends how your site is choosing to handle "login"