African lake malawi cichlids (mbuna) Or Lake Tangynikan cichlids... or dwarf riverine africans... or dwarf riverine SA/CA cichlids... ahhhh!
And oscars cant really be kept in a standard 55 gallon tank... a 12 inch oscar wont be able to turn around in a 12 inch-wide tank. And if its a wider tank then its only 3 feet long... and that doesnt allow a one-foot oscar much swimming space.