It's not only about "over filtration" at some stage here soon these fish are going mature and start either beating on each other or beating on their tank mate's if he's lucky/unlucky enough for them to pair up.
Even if he is "over filtered" in a 75 gl tank with 2 Oscar's he's going to be doing twice weekly WC of 75% nitrates will be thru the roof after a week.
In order for Oscar's to really thrive they need nitrates to be as low as possible, when nitrates get to 20 it's time to change a lot more than 15 gals of water.