O. Chem is actually a very easy class despite what you might hear. The main thing to do is just keep up on the reaction types and reagents. Spend some extra time on the mechanisms so that you can apply that to future reactions and understand how they work. O. Chem really only has a few basic mechanisms, but it has a lot of reactions. If you understand how the mechanisms work, then if you forget the reactions (and when you get to the final term you will have lost a few) knowing the mechanisms will make it relatively easy to work things out.