Make sure your water temperature is around 78 degrees.
Can you check your ammonia, nitrite, and nitrate levels?
Ammonia and nitrite should be absolute zero. Nitrite can be safe up to 40ppm or so. If ammonia and nitrite are elevated, you've got to do a decent sized partial water change.
Also, need some more info:
-how big is the tank?
-what specific filtering do you use?
-how big/old is the oscar?
-how often do you do water changes & how much water per change?
-do you dechlorinate the tap water during water changes?
Sorry for the questions, but these are just basic queries before anyone can help you. A couple years ago, my oscar was doing the same type thing from excessive nitrate build-up. I was a novice then so had to learn a handful of lessons in a short amount of time. fyi, Cell the oscar is going on 3 years old now.
