Rest has no direct relationship with 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").
They might as well have used Oauth for you to authenticate in a service that doesn’t use REST, and the reciprocal is true.
An example of this is the use of Google’s SMTP for "safer applications", which uses a variant called Xoauth2, i.e., 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 padlock in a closet instead of the gate. The relationship between the two is a mere choice of who made available.
A tip when 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 them using the correct name of both. Currently what is commonly used is Oauth2.
About REST:
What is REST and Restful?
In English, on Wikipedia:
Oauth
Thank you very much friend, clarified my doubts well.
– R. Zanel