My pursuit of ActionScript 3.0/OOP mastery has recently led me to these links.

Learning Actionscript 3 - Companion site for the amazing book of the same title. I recently picked this book up and can’t recommend it enough. If the O’Reilly books are a bit much for you right off the bat, this should be right up your alley.

goToAndLearn - Introduction to OOP - Learn the basics of doing object-oriented programming in ActionScript 2.
While I know this is for ActionScript 2.0, it is super helpful in that it clearly describes the use of classes and introduces some of the main principles of OOP.

Adobe Developer Center - Object-oriented programming with ActionScript 3.0 - Concise and extremely well written article discussing: What are classes?, Basic OOP concepts: Inheritance vs. composition, Encapsulation, & Polymorphism. Includes some extremely well commented source files - a MUST for anyone wetting their tip with ActionScript 3.0.