pffttt...IME it will be 50/50...have seen quite a few killer Oscars in 30 years....
+2
IME, oscars tend to be the aggressor less often than similar-sized CA cichlids such as jags, RDs, midas, etc., but when an oscar decides a certain area of a tank is its own it make no bones 'bout it. And when another fish gets aggressive, most oscars will give it right back.
To the OP, eventually you will wanna look at a larger tank. That in itself will help aggression as it allows more space for each fish. One option in the mean time is dividing the tank with an egg crate divider for a time. Allow each fish the opportunity to stake out it's territory. Make sure ya have a piece of driftwood, a clay pot or other decor on each side for each fish to claim as it's own. This may or may not help, I've had mixed success in the past. If the two fish start ignoring each other, ya can try removing the egg crate and see how it goes. Good luck!
WYite