Could be a lot bigger, but I don't see an Oscar in a 75g being a problem. Just don't go crazy with it a Rubberlip or a Synodontis or some catfish would add to the overall health of the tank, so it would be better to do a couple small-medium cichlids so you can have room for catfish and plecos. A fish in a tank is not always the best way to do things. I don't think any tank is FULLY-healthy without commensals and bottom-feeders. Shoot, even a Chinese Algae-eater....something to clean-up, maybe a few of them.
I would more likely do something more of a mixed bag, like a JD as boss, and some smaller stuff, a convict or 2, a Jewel or 2, FM, maybe a few Acaras or small Geophagus. Oscar's gonna bore the heck outta you in no time. A bigger-ish one 8-10", and a couple smaller cichlid under 6" and some bottom-feeders should live harmoniously as long as you make plenty of hide-spaces and caves . You can get rocks from a local stream or other CLEAN water source and submerge them in boiling water to sterilize them before using in the tank. Create a series of archways along the back of the tank big enough for each cichlid to hide completely out of the others' sight and try to make sure every cichlid has at LEAST 2, preferably 3 caves each so they won't fight over territory. Making them somewhat equal in size by comparison will help this as well. I like to mix PVC pipe elbows or joints/3-ways into the mix as well