JD Oscar: Don't panic, there's a huge team in fishlore ready to help you.
How big is your Oscar? I don't want to sound hostile in any way, but you will need a bigger tank for an Oscar. 55gal minimun if he/she will be the only one there. Don't worry about it now, just keep it in mind for later.
IMO you have a few options:
a) If your Oscar is still young (less than 9") I suggest you take it back to the
LFS while you go for fishless cycling (make sure the Oscar gets acclimated by being there while it's done, these guys lied to you in order to make a sale). They should hold it for you for a couple of weeks (purchase some large cichlid pellets and leave it at the LFS, visit daily if possible, even tip an employee). Young Oscars may not be as hardy as adult ones with water parameters (I'm no expert, just heard that info recently, and haven't confirmmed it yet).
b) Keep the Oscar in your tank, but get Prime to lock-up toxic ammonia, and add it daily as you do 50% daily water changes as Shawnie clearly says. Do not clean the filter media. Test for water parameters at least daily (I would do that twice, before and after water changes) with a liquid reagents based test kit (most use API master test kit, but there are other good brands too, like Tetra Laborett -this one does not measures nitrates but has
GH and KH reagents), do not use stripe tests, those lack the accuracy you need.
c) Get the Oscar to the LFS for a couple of days (again, be sure of acclimation), put Bio-Spira in your tank, and bring the Oscar back after at least 24hrs of adding the Bio-Spira. The water may look bad, but your fish will be fine. I have never used Bio-Spira, but have read a lot of posts about it, in situations like yours, Bio-Spira really works great (if not frozen), it may save your Oscar. If you go for Bio-Spira you may not need to add Prime (not sure but the veterans will know, for sure).
Good luck. We are with you in this emergency.
Pepe
Santo Domingo