Are there any types of moss that I could use when setting up my tank for an oscar? or would he just tear it all up?

Also, if I'm using gravel as my substrate, do I just lay the moss on top of it or what?

I know some members used those balls of moss, and the oscars left them alone.
You can also try Java moss, if i'm correct, and that looks beautifull when attached to a piece of driftwood with some fishingwire.
And it can't be uprooted.
Nevertheless, it's allways a risk, for oscars redecorate the tank, just the way they like it.
And your moss will not survive a treatment like this:

yeah i agree with noddy... moss and oscar do not mix