If you put them in tanks which had no fish in them before for at least a day, then the tanks you put the goldfish in would not have been cycled. If these "holding" tanks are also small (how many gallons do they hold?), then the sudden impact from the bad water quality conditions which will be taking place in the tank will be particularly devasting on the goldfish, which is probably what is killing them right now.
You should always have your own water quality testing kits which can accurately test for ammonia, nitrites and nitrates and ph, having such test kit/s is like having a basic first aid kit for yourself. Most deaths or diseases/parasites which happen in aquariums are often caused by stressful water quality conditions, the only way to know exactly what is going on in your water quality is to test it as stuff like ammonia is not visable to the naked eye etc.
Oscars do not need feeder fish to survive and thrive, and either way, if one chooses to feed feeder fish to oscars it is strongly advised that you do not buy store bought feeders (which more than often tend to be riddled with diseases and parasites, which can then be passed onto the predatory fish in question) or feed goldfish in general (which are rather unhealthy for tropical fish like oscars, since goldfish are very slimey and fatty fish which are not very nutritionally fullfilling etc).
Oscars are omnivores, so veg in their diet is also just as important for them as more protein orientated foods- to learn more on feeding oscars and keeping them in general, see the forums very own very helpful/informative profile on oscars;
http
/www.fishforums.net/index.php?showtopic=47035