your oscars shoiuldnt be allowed to get past the three inch mark in the ten gal. After three inchs you risk stunting the growth of the oscar. Also dont feed your oscars feeder fish. You didnt say how they got ich but im bettign you use feeders for food.
First they arent good for your fish
second feeders carry parasites like ich and other worse ones.
third if you are gonna use feeders you need a seperet tank for quarntineing them for atleast two weeks to make shore they arent sickly or carrying anything
try useing a cichlid pellets oscars will eat anything.