You could do CAs or SAs. You could have your oscar if you want, and then just stick to some smaller (6"ish) cichlids to live with him. There is a chance the oscar will be aggressive to them, but more often they are not so bad. Some geophagus would work as your other cichlids, or perhaps acaras or guianacara. Could probably do a green terror or severum as well, though they can get mean and may clash with the 6"ish fish, moreso the GT. Some spotted silver dollars or a pinktail chalceus could finish off the stock.
As for CA, the bigger fish tend to be more aggressive than those from SA (with exceptions), so for example something like a Trimac would probably need to be alone in this size. But you can keep convicts with *almost* anything, could have your big centerpiece and a couple breeding pairs of convcits. Or, a community of less aggressive CA fish such as thoricthys, cribroheros, etc. would work. Some of these get pretty big like 10-12".
These are a couple of examples of type of setup you could do, the fish can be changed with similar species for the most part. I also tend to keep livebearers like swordtails with CAs that aren't overly predatory.