When adding a new oscar (or any other fish that can be a bit agro) to a tank with a oscar in it, I have found the best method to take your oscar out of the tank and change the tank around (a plus, to swap some of the hidding places / large wood, rocks for other bits of similar size) and then introduce the new oscar/s and the same time as putting your oscar/s back into the tank.
I have found this way to work the best, a bit over the top but with a fish like an oscar better to be safe than sorry