In my opinion you are already fully stocked, you may actually need to get rid of one of the oscars if the other two pair up. You could go with some silver dollars just to get a little more action in the tank.
The max Oscars in a 125 should be three even three is pushing it. If I was you I would either get rid of 1 Oscars and the Mota OR 2 Oscars and get yourself some sort of dither fish. A catfish probably would not work well in there due to aggression and would add even more to your bioload.
i remember hearing somewhere 3 oscars is asking for trouble because 2 will always gang up one. never actually saw it happen. i would ditch an O and probably wouldnt add anything new.