This can be a problem, normally when Oscars get a little bigger. This is why giving them a varied diet is a good idea. To be quite honest, I couldn't tell you why they suddenly turn their noses up at a food, I don't think anyone could give you a conclusive answer. From all the fish I've kept, Oscars are the only fish that do this, all the other fish I've got will eat whatever you give them. Anyway, starving them is probably the best way to get them to eat something. Just remember that fish can go without food for quite a long time, a few weeks actually. So even if you didn't feed him five days, he wouldn't come to any harm whatsoever. I sometimes treat Oscars like little children, if they won't eat their dinner, they get nothing else. If he is a healthy Oscar, he will eat. Try dropping in one or two pellets, don't drop too many and that once. I think that sometimes making the fish think there's hardly any food can help entice them to eat.