In situations like this, starvation is often the best way to get an Oscar on to another food. I know it sounds cruel, honestly it is not. And Oscar can go without food for quite a long time, a few weeks actually. So even if you wouldn't eat for a week, he is not going to die. If he is healthy, he will eat. He will not starve himself to death just because you won't give him feeders.
Stop feeding feeder straight away, try a few pellets, if he doesn't eat them after a few minutes, take them out. Repeat the process every day. I'm sure that he will start eating them in the end.