If you have a basic understanding of OOP (I mean very basic) and want to learn more, and also apply it to ActionScript, check out this site. It is a great resource!
That site is great for Flash 5, but MX has some built-in functions now that make some of what is showcased on that site outdated. For example, FMX has a super() method built-in. Also, I have heard that the next version of Flash will have an extends() method as well (which means if you write your own inhertiance extension function I suggest you name it ‘extend’ instead of ‘extends’). The people at Macromedia have their own way of setting up inheritance, but there are a few issues with it. If you want a nice, clean inheritance system for FMX check out quantumwave.com.