I have kept several pairs over the years, and like them quite a bit. The male is generally significantly larger than the female, with the pronounced red hump. My males never got more than 5 inches, and the females never more than about 3. They are classic geophagus, constantly rooting through the gravel.
They spawn very regularly, and are not shy at all about it. Only mouthbrooder where I could watch the female release the eggs and pick them up in the middle of a well-lit living room. The females are excellent parents, and one problem I had was finding homes for all the kids. Another problem I had is that the males tend to breed the females to death. Since they are mouthbrooders, rearing young takes quite a bit out of the females, while the males are always ready. I would suggest several females, and even better, a separate tank to fatten up the female after the brood is taken away.
I have never kept oscars, but I figure the redhumps can take care of themselves. They are generally peaceful, though, and work well in a mixed tank with larger fish. HTH.