I haven't had many, but the oscar that I have now is very picky about his tankmates. It's just him and a common Pl*co(12"+) in a 90, they've been together since the start, and that's the only other fish my oscar(12"+) will allow to be in "his" tank. Everything else the oscar tormented until they were dead or I pulled them out and returned them, including some cichlids that are supposed to be more aggresive than an oscar. For some reason he lets the pl*co stay. I'm not sure if this is common, but its what I have experienced.