I know a little about these fish, and I also know their is very little information available on them.
I know that some morays are freshwater - whether yours is or not I can't be sure.
I know that some are brackish, and some are full sea water, and some can switch between the three (usually for breeding purposes as far as I am aware)
I did own one, and it died, but not because of the freshwater, but I couldn't feed it.
I strongly suggest that you either return it and save yourself the cost and hasstle of owning this beautifull fish, I suggest this even more if it fights with an oscar!
But if you are determined to keep it, you will need caves and dark places for it to hide and keep clear of your oscar.
you will also need to look it up as much as possible - perhaps Frank can help, I don't know how much he knows about moray eels.
You do need to find someone on-line who has had sucsess keeping these fish alive, as they will be the ones with all the answers.
I know I did something wrong when I had one, I am pretty sure it was feeding, I was suggested to use live comets (whatever they are? - live fish I think)
I would also try every possible food source I can think of, bait from the fishing shop, worms, small fish, squid, seafood, and more.
I would not trust the person in your local fish shop - they sound very dodgey to me
and these fish are definatly not for newcomers to fishkeeping