I'm pretty sure our rescued oscars and pacu came out of this WalMart tank.
There probably was a time when our oscars were bigger than the pacu. Now, our oscars wouldn't think of messing with their pacu brothers. It would be like a row boat versus an aircraft carrier.
I myself have a 24 inch pacu that got bullied pretty bad by a 11 to 12 inch Oscar.....I had to seperate them.....I have seen my pacu push most ne thing out the way-But he wan't willing to mess with that oscar.....
Both fish are fast growers. But the oscar will pretty much stop around a foot. The pacus will slow, but will keep growing. If the pacus are continually harassed, a divider may be necessary. How big is your tank?
I don't know. Guess it depends on the fish. My pacu was 16" and got along just fine with all the osacars. Even my breeding pair of oscars left him alone.