You need a 75-gallon tank for two oscars. Oscar fish are quite large, with the average oscar growing to be around 12 inches. Some oscars can even grow to be 20 inches in length. Oscar fish are also territorial, so a small, overcrowded tank will cause your oscars to fight and injure each other.
The territorial nature of oscar fish is why a single oscar requires a 55-gallon tank or bigger. For every extra oscar, you need to add 20-30 gallons to the tank.
Moreover, oscars grow rapidly. They have an astonishing growth rate of almost an inch each month for the first seven or eight months of their lives. Due to this rapid growth rate, they can’t be kept in a smaller, makeshift tank as juveniles, either.