Object Oriented Programming in itself is not hard.

The hard part comes in doing it well. Where to put the cut between code so you can easily move things to the common base object, and extend them later? How to make your code usable by others (extend classes, wrap in proxies, override method) without jumping through hoops to do so.

That is the hard part, and if done right can be very elegant, and if done badly can be very clumsy. My personal experience is that it requires a lot of practice to have been in all the situations where you would WISH that you did it differently, in order to do it well enough this time.