Hi, I'm a newbie, in my limited experience, this is what I consider you could do:
If injures are really bad, and it's within your possibilities, consult a veterinarian.
If you have a Hospital tank running, get the injured Oscar in it, turn the lights out, provide as good as possible oxygenation -if you have HOB in it, lower a bit the tank's water level to create shallow water for your fish; injured Oscars tend to lay down, maybe even tilt a bit, don't worry about it, it's normal behavior when stressed; and keep stable warm temperature.
Otherwise, if you must keep the injured Oscar in the actual tank, secure that side of the tank -cheap egg crate, used for cover in fluorescent ceiling lamps would work fine, just perform some drilling for water circulation, and provide as good as possible ideal water conditions.
I believe Oscars tend to heal from fight injuries quite fast. Unless your fish show signs of bacterial infection, don't jump the gun for antibiotics.
In the meantime provide a high-protein diet combined with spirulina based food -to boost tissue replacement and help immune system, just feed sparingly, several times a day -to keep optimum water conditions.
You may have two Oscars that won't be compatible to share a tank. This is not unheard of, although not too common either. A radical re-arranging in the tank aquascaping might help in the future -e.g. adding two or three driftwoods, stones, changes in substrate... anyway let's focus on healing your Oscar.
Hope your injured pet recovers well soon.
Pepe
Santo Domingo