You can have two oscar fish in a 75-gallon tank. Oscar fish can grow to be as big as 16 inches long, and a full-grown oscar needs a 55-gallon tank or bigger. For every extra oscar fish, you need to add at least 20 gallons to the tank. Thus, you can keep a pair of oscar fish comfortably in a 75-gallon tank.
With that said, if you can get a 100-gallon or bigger tank for your pair of oscar fish, go for it. Oscar fish are territorial and aggressive, so providing them with more space will prevent them from fighting each other.