Many cichlid keepers who raise fish such as midas, trimac, carpintis, synspila, oscars, and other species similar in size and temperment use the 3' - 65gallon tank. 36"Lx18"W. Good foot print for a single species. A single adult oscar would work out fine in the tank providing decor is minimal and filtration is up to par.
Ex. I myself have a Perfecto 65gallon combo(tank, hood, lights, pine cabinet). I filter it with (2)AquaClear 500 Power filters, and heat it with a 300watt titanium heater.
If space is available a standard 4'-75gallon is my prefered minimum tank size.