Dempseys are not aggressive per-say, more defensive. I have kept jd's in with all my oscars over the years. The only thing I would worry about would be size differences. Young fish are not as tough as older ones.
That is the big issue here. We know neither fish nor tank sizes. From the standpoint of fish size, jacks and oscars do get along.....when the oscars are bigger than the jacks. We only know that the oscars are smaller than the jacks. And the OP has described the jacks as one large and one small. This likely means the large jack is quite a bit bigger than the oscars. And jacks are still aggressive. Whether it be aggression due to defensiveness or offensiveness, it doesn't matter. One wrong move on behalf of the oscars and there could be severely damaged or dead oscars. Unfortunately, until we know the tank footprint and actual size of the fish, not everyone will likely agree whether this will work or not.
