I shouldn't worry about it, if I saw Oscar for sale in the supermarket, I might just buy some just to see what it tastes like, it's not as if you are taking your own pet out of your tank and eating it.
You certainly can ship fish overnight, I have bought fish over the Internet. However, I wouldn't like to advise you on the best way of doing it as I've never done it myself.
I better just add that this isn't the right place for posting threads about Oscars, this really belongs in the "Oscar forum"