First of I feel your pain.I do wish you had been a member and been able to ask about the capatability of these fish before you purchased them.
I don't know the condition of the corpse of your oscar. nor do I know the size. I would, probobly, to be on the safe side recommend a 50% water change.
I do not wish to admonish you, especially in your state of anger and confusion, at this time. However, I would like to offer some friendly advice. Fish compatibility is a key issue when keeping any fish. Larger fish always pique our interest. Oscars, Red Devils, Jags, all are very neat fish. However, they are also very large and aggressive fish. Thier size alone should tell us that there are special conditions to be met and the threat of them doing damage to others is a possibility. Please, in the future, research before you buy. Find out the nature and needs of the fish you wish to buy. They may be the most beautiful fish God evr created, but if he's a killer, he's not the fish for you. Names like Red Devil and Green Terror are given to these fish for a reason.
And, truly, I have been where you are. And I do feel your pain.