Try some large catfish like channel cats, or maybe a TSN catfish. I have an 8" TSN. I had to move him bacause my Dovii, and Red Devils were picking on him. TSN cats get big though. I have seen one at a pet store in Fayetteville NC, he was between 2 and 3 feet long in a 300 gal tank.
stuff that can't fit in his mouth. other than that, oscars are generally peacefull, its usually the other fish that you have to be worried about getting along with the oscar.