My oscar just has Hikari Gold as it is the only thing he will eat, I'm not happy about this. He was bought up on lots of different foods but he gradually went off everything but the pellets. Actually, he does still eat the various moths, spiders etc that I catch around the place but any fresh fish, shrimp etc he ignores.
Oscars can be the very devil for doing this. For his own good, you might well have to 'starve him into submission'; keep offering the healthy foods and do not give in and give him pellets. This may take a couple of weeks, but you will get there. Sadly, I've found most oscars are far more strong willed than their owners, so they usually win and get given pellets, but it's not a good diet for them, as you're aware! My last oscar went through the same phase; he used to swim up to the food, gaze at it mournfully, then turn away and lie down on his side as if he was dying, the drama queen (he used to do the same performance at water change time as well
).
I feed most of my tanks once a day, when I switch the light on. My staple food is a high protein granule or crisp (I've never had fish that liked flake), plus meat based catfish pellets and the occasional algae wafer, blanched veggie, frozen bloodworm or brine shrimp. I also have freeze dried daphnia, as it's one of the few things that stays floating for long enough for my marbled hatchets.
The 55l cube that sits next to my bed (
) has some really small and shy dwarf emerald danios and shrimps, so I feed those a couple of times a day; once along with everyone else when I put the lights on (usually around tea time) and again when I go to bed to read, so I get to see them properly