Rest has no direct relation to OAuth, the first is an information exchange architecture, and the second is an authorization mechanism (actually, ends up being confused with authentication , but this is worth a separate" debate ").
You may well have used OAuth to authenticate yourself to a service that does not use REST, and the reciprocal is true.
An example of this is the use of Google SMTP for "safer applications," which uses a variant called XOAuth2, meaning a new authentication for an "old" protocol.
Just to compare (exaggerating a little), your question would be "What is the relationship of my padlock Pado with the gate of my house?" "You could have used other padlock marks for the gate, or you could have used this same lock in a locker instead of the gate." The relationship between the two is a mere choice of who made it available.
One tip in using and studying OAuth is to understand that OAuth and OAuth2 have significant differences, and you should focus on the correct documentation, and always refer to using the correct name for one and the other. Currently what is commonly used is OAuth2.
About REST:
What is REST and RESTful?
In English, at Wikipedia:
OAuth
