That's 33 US gallons, right? Assuming you cycle before hand, and you're starting with a baby oscar, you should be okay for 8 months MAXIMUM. That means you may be changing water multiple times a week in a few months, it also means you MUST not add any new fish to the 33 gallon tank with the Oscar. Not only because of the bio-load, which will be minimal if you start with a baby oscar, but because Oscars are territorial. As long as you're sure you can provide at least a 55 gallon tank in 8 months, (though a 75 would be better) and you are committed to doing enough water changes to keep your nitrAtes below 20 ppm., then yes you could keep a baby Oscar healthy in a 33 gallon tank. (for 8 months)
Obviously, if the Oscar you want is already an adult, then it won't work. You could not keep a 33 cycled with a large Oscar in it. My Oscar is about 9 inches now, and his "droppings" are the size of my smaller fish!! (different tanks, obviously)