First questions/comments:
White stuff sounds like a fungus (if anyone disagrees, please speak up. I've never had a sick fish). You'll want to treat it for that. If you have snails, get them out of the tank, because the most effective medicines will kill inverts.
Get yourself a water
test kit. the Master
API kit is the favorite around here. (I use the strips, but only because I hate to be wasteful. Once I go through my bottle, I'll be switching to the API)
Test for
ammonia,
nitrate,
nitrite,
pH,
What are the readings?
Ammonia and Nitrite should be 0, or close to. Nitrates should be, at the very most, 60 or under. 20 or under is what you're shooting for. (the numbers I'm giving are
PPM)
Have you cycled your new tank? It could be that the tank built up too much toxins before the bacteria colonies could get set up to take care of ammonia and nitrites. If either of those numbers is much higher than 0, there's your problem. We can work on getting your fish through getting the tank cycled if this is the case.
Do you use municipal water? If you do, what conditioner do you use?
While it sounds like a fungus is killing your fish, the root of the problem is more likely "what stressed your oscar to the point that his immune system couldn't fight the fungus?" Any of these could stress the fish out.
If you can get to a pet store tonight (the tonight is presuming you're in the US), do so. I'd get something to treat a fungal infection immediately and start treating the tank.
Good luck to you and your oscar. I hope he does well.