Oscars are near impossible to sex. They are one species of fish that dont have any visible variations to determine sex.
Well, they do have visible signs, but they are hard to see. You can try giving them a lot of food and see if the female's .....parts stick out (forgot the name)
One sure fire way is venting, but i dont recommend that unless you really know what you're doing.
You most likely wont be able to tell until they are mature and start displaying spawning rituals. Even then, two females will sometimes display like a mated pair.