Oscars eat anything really.
Crickets, earthworms from the bait shop, grasshoppers, mealworms, superworms, tilapia fillets, oranges, zuchinni, peas(highly suggest this it’s a natural laxative), FD treats, frozen treats, carrots, etc
No need to get real fancy. A good quality pellet, some peas a few times a wk, & replace a meal once in a while with some twitchy insects will be more than enough. Anymore than that & they’ll reject pellets then you have a battle of wills till it eats pellets again. At 10in + you can feed them 5 times a week no problem