I kept Spotted Gar and Tiger Oscar together without any problem. I never seen them fight or picking on each other before. Gar is about a foot long and Oscar is about 5 inches. What I think is that Oscar won't bully or pick on fish that is twice their size. Well, I don't know, I think it depends on what kind of fish you are housing with Oscar I guess. If Gar and Oscar were the same size or Oscar is bigger, maybe Oscar will do some damage to the Gar or totally kill it, depends on how aggressive Oscar is. From my own experience, you can safely house them together but you gotta make sure that your Gar is at least twice bigger than your Oscar.