A single oscar would be ok in there, although bigger would be better. I would avoid the balas as they can get over a foot long and are pretty fast swimmers, so they should have a tank more than 12-13" wide (young balas would be fine in there for a little while though).
If you went with a little bit smaller cichlid like a severum, there would be more options for tankmates-especially because severums have smaller mouths, so you could keep some of the larger (as in around 4" or so) tetras and barbs or smaller bottom dwellers like some cories with them while an oscar might be able to eat some of them
If you went with a little bit smaller cichlid like a severum, there would be more options for tankmates-especially because severums have smaller mouths, so you could keep some of the larger (as in around 4" or so) tetras and barbs or smaller bottom dwellers like some cories with them while an oscar might be able to eat some of them