I want to make an app that needs authentication, I want to use Oauth2 and I have my own REST API.
I know this is very brief question so I'm not expecting direct answer.
If you have any links or articles for this it would be helpful!
Using a library such as react-native-app-auth will help you a lot. There's quite a lot of setup to do which is explained in their documentation. Feel free to update if you run into troubles using it.
Go through this blog: https://medium.com/@alexmngn/the-essential-boilerplate-to-authenticate-users-on-your-react-native-app-f7a8e0e04a42
it is very easy to understand and shows basic authentication by OAuth2.