Hi Hump,
My first experience with Oscars was much like yours. First off I'd be worried about if your tank is cycled or not, just running it for a week won't do.
I hope I explain this correctly. Your fish will create ammonia from it's waste, and uneaten food decaying. You need to have established bacteria that will change that ammonia into Nitrites and other bacteria that will change the Nitrites into Nitrates, and you will then do frequent water changes (at Least once a week),to remove the Nitrates.
It usually takes awhile (often 4 or more weeks), to establish the bacteria. There are a lot of posts on this forum on how to cycle a tank, products you can use, etc.... I've only done it twice and the second time I used filters and media from a established tank to get it going, so I'm no expert.
If your tank is not cycled your fish is Greatly at risk of dying or suffering long term from it.
I would strongly suggest taking your fish back to the store and then spend some time looking through this site to learn more about getting your tank set-up.
I hope I don't come off as mean but I learned the hard way, I bought my first O from a similar store and they didn't tell me anything about cycling the tank, etc..and I was clueless and lost him.
Oscars are amazing pets and I love mine to death, but I wish I had taken the time before hand to find out more about them so that first one hadn't suffered and died.
If petsmart won't take it back try a store that specializes in fish only.
But seriously check-out this site! The people on it are Great and Very knowledgeable!
Oh, with food only give them what they will eat within a couple minutes and remove what ever they didn't eat.
Good Luck!
-Az