It's difficult to grasp Object Oriented Programming (OOP) unless you have a good understanding of Object Oriented Design (OOD). I'd recommend starting by studying OOD. There are lots of good sources, both in books and most likely also on the 'Net. Once OOD is understood, OOP will become no more than an implementation, almost a question of syntax.
You should probably know the subject in-depth before giving classes. But of course, that's your own decision. You know your own capabilities in teaching subjects you don't really master a lot better than I do. It was just a thought.