Oscar breeding can be a brutal thing to observe. It looks like a fight when it isn't. Typically, the spawning ritual starts when the fish lip lock. This is them testing each other for suitability for mating. Gill flaring is usually more a fighting stance than a mating dance. Tail slapping is also more fighting. Considering that there are not many hiding places in your tank, this may be the new one trying to assert it's dominance over the larger fish as a means of protecting itself.
If the 2 are preparing to spawn, you should see them both scraping the surface of the spawning site ( probably that flat rock on the right). Oscars can lay upwards of 1000 eggs so the site they choose is usually a large rock or one with a lot of surface area. If neither one is cleaning a site, I'd be prepared to separate the fish with a clear divider in the tank so that they can get used to each other being there without being able to hurt one another. Attacks to the body are more from fighting than mating. Facial injuries are more from mating than from fighting. I've had many pairs that were missing lips or were banged and bruised about the head but their bodies were perfect. As I said in the beginning, breeding oscars can be brutal. It's all about observation.
Hope this helps