African may eventually take out the oscar as they will randomly attack tankmates and are best kept alone. assuming you most likely have Protopterus annectens (west African lungfish), i would go with a 125-180g tank. they don't need a ton of swimming room, but should be able to at least stretch out. a 180g should house the fish for life, 120g wide (2' wide tank) should house it for many years. good luck--
--solomon