jacks can be good mates, but they can be too aggressive for o's at times. they grew at about the same rate until my jack killed his oscar tankmates years ago. all was well up until the fish were about 11" and the jack was a tad smaller. i had kept jacks with os b4 that with no problem.
in a 55 gallon tank, your sure to have territorial problems as well as water quality issues. but for now, they are good mates.
stay away from feeding live fish, its said to make fish more aggressive.
if you dont rehome 2 of the fish, or get a bigger tank, theres a good chance you'll have 1 jack with no oscar tankmates.
just to show exactly how aggressive jacks can be, after the os died i bought a small jack to grow in a 55 gallon tank untill she was big enough to add into the other. i added her in at about 4.5 inches and she did fine for a bit(hid an all, but lived comfortably). the male would chase the female around sometimes but when the smaller retaliated she usually won. at about 5.5 inches the smaller jack (female) would occasionally beat the hell out of the much larger male. no serious injuries would happen, but close. awhile later they ended up pairing but they hated eachother for quite some time. so keep an eye on that jack!
do you think a jack dempsyis compatable with oscars? 
