I'd say that one a 90 is more than enough for one Oscar, as long as you had the right filtration, you could probably even get away with adding a dither or two; but I do agree with the post's above that 90 is too small for 2.
I see them smashed togehter with other fish in 55's and below all the time, sometimes the people even manage to keep them alive in these crowded setups; I just wouldn't count on a very long or comfortable life for the fish though.
I'd recomend a 125g if you did decide to keep the 2 though.