I've just completed a really nice SDK for iOS. A major functionality handled by this SDK is the sign in through OAuth. To let user log in I present an in-app browser (SFViewController) presenting a login page that is hosted on my server... My question is, how can I perform OAuth login using tvOS... if I'm not wrong, I cannot present a webview or open an external browser right?
You are right, there is no web view or external browser on tvOS. If you must use OAuth, follow the ideas in the answer to this SO question: Authentication to dropbox on tvOS - redirect your users to the web or to a companion app, that does the authentication on behalf of the tvOS app.