Some people believe this is cruel. Also depends on the fish. Goldfish are quite unhealthy as they are rich in fats. There's really no point in using feeders. A high quality pellet along with other foods as occasional treats is all you need.
I could not agree with the Nutcase more. I would try not to use cold water fish as feeders as there is a higher fat content in these, which over time can lead to health problems for your Oscar. If you purchase feeders, it's best to quarantine them to ensure maximum health. I also feed my feeders just prior to adding them to the big tank. It's just an extra method of adding nutrition to your Oscar's snack.