What to put on a oak tree branch after trimming?

My arborist will not prune an oak during growing season. In fact, my oaks were pruned in December (we live in Chicago) to prevent any disease from entering the wound. If you are in the Midwest or a cold climate, don't trim any oaks until they are dormant. If you must trim "in season", I would contact an arborist (perhaps your city has an arborist on staff) for the proper sealant.