Check all water perimeters to make sure everything is balanced and ideal, bump the temperature up to 80-85 [this will ensure the Piranha eats the Oscars.], provide shelter/hiding spots and give him a week to adjust to his new home. If he still doesn't eat than soak a peeled shrimp in garlic water for about an hour and drop it in the tank.
I'll bet money if you follow those steps he'll eat. He's probably just adjusting to his new home.
