Don't do anything until you know what is wrong. Again, check for ammonia, but also make sure your pH is stable. I had the worst time with Oscars until I got my pH stable. My O had cloudy eyes until I got the pH buffered a bit.
He could also just be 'zoning'. Most Oscars tend to drift in and out of a sleepy state throughout the day. One minute they are begging for food and then next, they turn green and stare into a corner of the tank like zombies. If this has been going on for days, it could be a problem, but start with water chemistry first.
Oh, Oscar specific Q's: Have you added any new decorations? Changed his routine, etc.?
He could also just be 'zoning'. Most Oscars tend to drift in and out of a sleepy state throughout the day. One minute they are begging for food and then next, they turn green and stare into a corner of the tank like zombies. If this has been going on for days, it could be a problem, but start with water chemistry first.
Oh, Oscar specific Q's: Have you added any new decorations? Changed his routine, etc.?