Just wait till he goes in the big tank. He's probably claiming most if not all of that tank to himself and there will be disputes with whatever you put in there.
Its recommended that 1 Oscar have at least 55 gallons by itself, so you could ad maybe one small tankmate but like rallysman just said, The Oscar probably will claim the tank for itself.
When left by themselves Oscars typically view the entire tank as there personal territory, and will not take kindly to new tankmates. Wait untill you put the Oscar in the large tank, then get some Monsters
if you decide to get a new fish my recommedation would be to get either a bottom dweller(catfish) or a top swimmer(aro) so they do not share the same section of the water column...that would cut down on territoriality and spread your attention in your aquarium.
