These grow quite a bit bigger than the oscar- sizes of 30 inches and above are mentioned by most authorities, and I have seen some massive ones in public aquaria. You would be looking at a tank of several hundred gallons. They are mainly herbivores but will eat smaller fish and have been known to take people's fingers off. Apparently absolutely hopeless to get rid of once they have started growing big, as public aquaria are already up to their necks in them.
If you still have a 10 gallon tank, with maybe a possiblity of a 55 gallon down the line, I can't help feeling that you are looking at the wrong end of the spectrum- you should be investigating tetras and small rasboras instead. Though not as tankmates to the oscar, evidently....