I'd leave the divider up for a while get them used to seeing the aro , your aro needs to be much bigger to tackle a pair of angry Oscars in an established tank my guess is 12-14" so it can take a few hits a fight back , you could also try taking the big Oscar out and trying the aro with the smaller one at first , let it get established and put on some size then try big Oscar again.
Unfortunately it's pretty much always a case of trail and error with aros and big cichlids untill you find some balance , I've lost loads of nice fish over the years due to this problem , best of luck hope they work it out eventually