As BerkReviewTeach stated, there really aren't any mnemonics. The best bet for O-chem is to learn what makes a good or bad acid, base, electrophile, and nucleophile. Also another helpful thing to know is: A chemical reaction is the movement of electrons from electron density to electron deficiency resulting in the making and breaking of bonds. If you look at most of the reactions they can usually be solved from this and a little knowledge of resonance, Sn rxns, and E rxns. Of course, it can get much more complicated, but on the MCAT it will probably be basic O-chem that just looks hard. Knowledge of functional groups is also good.