You need a 55-gallon tank or bigger for an oscar. Oscars grow to be very large, with the average oscar growing to around 12 inches long when mature. Some oscars can even grow to be 16 inches long. For that reason, an oscar needs plenty of space to live comfortably.
A 55-gallon tank is a minimum requirement for a single oscar fish. If you wish to keep more oscars in your tank, add around 20-30 gallons of space per fish.
If you can afford to get a bigger tank, you should do so, especially if you decide to get tankmates for your oscar. Oscars are territorial, and small, overcrowded tanks will stress them out and provoke their aggression.