Welcome to the world of the Oscar fish

This is something that is quite common, I cannot honestly tell you why they do it, just sometimes they seem to suddenly go off the food for no reason. Obviously the first thing you must rule out poor water quality problems, so test the water for Ammonia, nitrites, nitrate and pH. I would suggest that you don't even try and feed them for a couple of days, you may then find that the love for food overcomes stubbornness and they start eating. That is not much more I can say for the moment, so see what happens in a couple of days.