Here’s two diagrams i found describing the OAuth Workflow.
The first is from dev.twitter due to @idangazit. I found the second on a Japanese website, I believe it is sourced at a VERY COOL website that lets you create UML Sequence Diagrams online, WebSequenceDiagrams.com.
Here is the First from dev.twitter due to @idangazit.
Here’s the second from WebSequenceDiagrams.com