you should probly go with a few bichirs and 1 or 2 more oscars but i would be pushin it reason being the will only be housed in a 75...and most of the fish listed above are great ideas but require bigger tanks and massive filtration
Trade in that O for the fish you want, my suggestions would be:
Erythrinus Erythrinus,
Black Ghost knife ( or White Ghost knife if you can find any),
P. Delhezi,
Synodontis Eupterus
Synodontis Notatus
Pimelodus blochii
there are lots of fish that could fit in a 75g and be happy..
but you are very limited if you keep that O, he will eat everything that you put in the tank that fits his mouth.
im planning on building a monster plywood tank in the near future to put in the basement. I got a monster pleco, hes a perty little feller. I found some large 10" polypterus of some kind for $40 a piece. they were nice. I didnt but them though, instead I got the sailfin pleco.