I agree bigger is better. I think a 125 would be much better for an Oscar. To me, if somebody already has an Oscar and a 75 gallon tank, that's not the end of the world. Though, if somebody was asking for advice and did not have the Oscar, I would advise another fish for a 75 that stays around 10" and under.
Different strokes for different folks, though I think anything under 75 is just very obviously too small