Not knowing more either but I wanted to ask if you cycled the tank before putting the Oscars in? Would be really good to test the water or at least have the shop doing it and tell us the results, as pooky said. If you didn't cycle it first, you might be having high ammonia or nitrite levels; maybe one Oscar is just more sensitive?
Also, are they getting along well, or is one chasing the other around?
If there's a way to increase the surface agitation (with adjustments to the filter, or an airstone), I'd do that.