I know a lot of people do it, but to me, oscars and JDs should never be kept in the same tank.
JDs are from high pH, high mineral content waters in Mexico, and their normal territorial aggression level is on much more heightened level, they often come from and defend areas from, and where few other cichlid species are present.
Here is an example of the water parameters in their area.

Oscars are on a less much territorial plane and come from neutral to low mineral content waters from S American where many other specie of cichlids are present, so a much more variance of tolerance,
Below the type waters oscars come from

One other thing to consider, a 6" JD will often be much older than a 6 " oscar, so the JD may be at the height of its territorial needs, and experience as an individual
A 6" oscar is just out of beyond a juvie, and not yet able to handle the much more territorial adult.