Concept of OAuth login - Step after the access token

Hello Lovely CI Community,

I studied how OAuth (Facebook/google login) works in order to implement it in my project but i don't really get how it can exist beside classic method for a membership area.

Imagine this forum for example, you create an account with a classic register form, the system will create an entry for you in the DB with your login/password.
The entry can contain all your preferences and be linked to other datas. It will check the login/password each time you want to connect.

But in the case of OAuth, how it works ? Because the user didn't create any account and you just received an access token...
How can you deal with it in order to "get him into the boxes" ? In order for the OAuth path to join the classic one at a moment during the process ?

Not sure if my post is very clear... It's the step after the system received the access token which is blur in my mind. 

Thanks a lot for your help !