I'll give my standard response here and recommend you stop using feeders altogether.
Oscars are omnivores, and there are many things you can feed them...try and offer different varieties of thiaminase-free foods, like hikari ciclid pellets, worms, cooked peas, insects, etc. If you would like to include fish try frozen oceanic fish. There are a lot of options readily available that involve less risk and are far, far more nutritious. In the wild their main diet is insects so you generally can't go wrong with that (although if you catch them yourself like I do make sure you know what it is and it's not potentially poisonous).
Oscars are notorious for going from hogs to all of a sudden becoming picky eaters and going on 'hunger strikes,' and the best way to get them eating again is a variation in diet and withholding food for a day up to several days. They can go a long, long time without food so its not a health issue. I usually have at least one day a week I don't feed my O at all. After that you'll see them come zooming over to you when you come in the room, opening and closing their mouths and generally 'begging.' They get good at that and after a while they figure out they can train you to feed them if they beg. Remember to keep feedings on a schedule though, as they will eventually beg all the time, even if they're not necessarily hungry. Less is more in most cases i.e. several smaller feedings a day as opposed to 1 or 2 large ones.
Thing is, if you get them a 'treat' that they really like but don't use often, and you feed them that and that only for a few days until it's gone and then go back to pellets they can get snarky about it. It's like feeding a child McDonald's for a while and then telling him they have to go back to Brussell sprouts. There's going to be a sulking/refusal period. You have to think of them as petulant little kids sometimes....cause that, to me, is pretty dead on in how they behave.
You really want pellets to be the 'staple' diet, so I would attempt to use those consistently in some capacity. I always use pellets in the morning on a schedule, and then whatever I have else in the evening.
You starve him for a few days he should have no problem gobbling up the pellets, eventually.