Lay off live fish, that's the easiest way to pass on the disease to your Oscar. I would only ever advise someone to feed live fish if they breed them themselves.
Okay, you of see can't give me any information at the moment. Have you got any test kits? These are vitally important, it will tell us exactly what the water condition is. Oscars will become ill if the water is not in good condition.
Like I said before, we are stuck between a rock and a hard place at the moment, I couldn't possibly try and diagnose a health problem without knowing those readings. If you can do them as soon as possible and then post them, we can try and help. I have to warn you that fish are notoriously difficult to diagnose so I can't promise a successful diagnosis.