Try a bigger Oscar, might work out, but then you'd need a bigger tank. Do you plan to upgrade this tank or keep this stock in this tank for a good while? If you upgrade, you have more options... Other than that, firemouths usually cohabitate OK with dempseys. Maybe convicts? Could definitly do a mid size pleco. Look into semi-aggressive or aggressive, medium, South American cichlids. Most any of them should work.