AS 3+AS 2 in the same file

first of all, I don’t have flash 9(I think it’s CS3, right?) yet, so my questions might be a little bit dumb:

Is AS 3 fully OOP? If it is, does this mean that in an AS3 flash file, I can’t use an AS 2 style of code? For example, can I use : _root.mc1._x+=10 directly in a flash file(that uses AS 3) in one frame without creating a class for it? If I can’t, will it cause an error, or will the program ignore it?

thanks