I would recommend live food feeders as I fed mine on it for 4 years. King meal worms might make a good choice for a clean food as they are so easy to raise by the thousands and cheap too. If you want places to get them or instruction let me know. I took 50 of them and have turned them into 3000 in less than 5 months. Cost to raise, about $10 a month.
As for pellets and other foods, I can't say because the only oscar I had lived for 4 years and was about 3 pounds, he was 9 years old when he finally died and he was never fed anything but live feeders, goldfish, guppies, earthworms and crawdads. I got him at 4-5 years old from someone who fed him strictly on feeder goldfish. I think he finally died because he ran out of space in the 55 gallon he was in. There may be some really good pellets and other foods out there but I don't know anything about them.