Welcome to MFK, where pushing to new limits is something you see pretty often.
Okay I have 3 Pacu, when I got them I knew they grew pretty large & fast, but I have to say PACU ARE THE FASTEST GROWING FISH EVERRRRRRR. Their WORSE then RedTail Catfish NO LIE & I would know i've had plenty of both species. Pacu can be very aggersive, all Pacu go through a stage before entering adulthood, they become EXTREMELY MEAN they will usually pick a fight with any fish, mostly other Pacu they end up ripping each other apart, I wasn't prepared for that.... I had to put them in their own tanks for a while. But back to what your worrying about PACU & OSCARS. THEY GO HAND & HAND, they get along pretty easily. You have nothing to worry about. EXCEPT: If you ever decided to get another Pacu & put it in the same tank as that giant you have, don't be surprised if you see another side to your Pacu. Pacu will bully a new Pacu that enter in it's tank. But if you have a very large tank or even a pond you will most likly have no problems.
If you have any questions or ideas or anything don't be scared to ask me, just post a message on my wall.