How big is your Oscar? My large pair is about 10 inches and nothing bothers them. My smaller pair are 5 inches and 7 inches and get bullied by the Dempsey if I don't rearrange the tank often. I've tried convicts with my Dempsey but they fight constantly. It's really trial and error with Oscar tank mates. It breaks my heart when I wake up to nipped fins.
It all depends how much stress you want to put your oscar under and if you're able to rehome the tank mate if it doesn't work.