OAuth can be anything you want it to be, the standards are lax and give you plenty of room for getting the right implementation for your system. However you proceed, though, you'll need to check an access token on every request - and in a Slim application, a middeware can help enormously since it hooks in to every request by design. I've recently implemented this and thought I would share.

Beginning August 16th, Twitter will no longer support the basic authentication protocol for its platform. That means the only way to authenticate users will be through a Twitter application. In this tutorial, I’ll show you how to use Twitter as your one-click authentication system