I'm not saying not to check it. I'm saying it's highly unlikely. And the fact that it only happens at night, and other fish show no symptoms is another big indicator that water quality is not an issue. Another big indicator is that it sounds like the oscar has been doing this for a while. I would say 2 or 3 weeks. Enough to clear up ammonia, or kill the fish.
Overfeeding itself does not cause ammonia.
You took my comment that "someone else mentioned it" the wrong way. I was simply stating that another member mentioned it to give them some credits. I do this often, don't get offended.