I agree, Oscars are too aggressive and most likely will pick on the rays and eat most of the food leaving the rays underfed. Blackworms tend to burrow in the sand and the rays will get them as they forage but you can't control how much each ray eats. When my rays were small I primarily fed them frozen bloodworms until they put on some weight. Then I switched to nightcrawlers, silversides, and pellets. Nice rays! Good luck!
