I put my oscar into my established tank when he was 3" long that included a 6" rts, a 5" featherfin cat and a foot or more long pleco.
A few months later he is 5-6" and living only with the pleco and the catfish. I moved the shark to another tank as you could tell he bricked it anytime he was close to the oscar although I never saw any attacks. Then one morning the shark had taken a beating, cut in a few places. I suspected the catfish until I saw this attack so I moved him out and he has recovered completely now and back to his old self.
Oscars are bad-ass. Anything that scares my rts is bad-ass. Featherfin and pleco seem to work fine and they are not bothered by him in the slightest.
Catfish are usually good suggestions. But I was hoping someone could suggest a decent fish that are very passive, that get good color and size and aren't bottom dwellers.