A nice Green Terror or JD would work well. (That's what I have.) What you can also do is let the O establish dominance by putting him in a couple of weeks before and then add even a manguanese and the Manguanese will not be very aggressive.
i agree if its bigger and they dont fight, but theres lots of odd balls that can go in as long as they get big probably better to go with fish that wont even pay attention to your oscar ie plecos i have a ornate bichir in with my oscar they get along great, dont even notice eachother.
yeah thats the thing it just depends
and oscars are more timid in groups
they get more teritorial by themselves
also it helps to get the fish you want while your oscar is still little