Oscars are generally aggressive and some times very unpredictable fishes. It is hard to manage when it suddenly decided to whack another fish for no reason?
Keep a close watch. If the oscar continue to disturb the gar, you will have little choice. That is the reason I sold my big guy away cheaply although I really like him.
thanks for the comments everyone!! unfortunately i had to move him out of this tank, waiting for the new tank to arrive before adding him in w the rest again.