Does this new Oscar make a total of 3 in the tank now?
Some techniques I have used...
What is the current temp of the tank?
I have found keeping the temp around 79 to 80 tends to keep my Oscars (2 tigers in a 125gal) more "mellow"
Keeping them well fed (whole krill, pelleted food, etc.) also seems to keep the aggression down.
After they matured and started sparring, I used to stand by the tank and would break up the fight with a pair of tongs that are normally used to arrange tank decorations. It seemed at some point, they just stopped fighting - or else the tongs would come in and chase them.
Try less decorations in the tank, then no fish can claim a territory. Carefully observe them after this, as it could make things worse for all tank residents.
Try less light on/in the tank or shorter light(photo) period.