I agree with batman, I have an Electric blue dempsey that I bought over a year ago at two inches. It is now approx seven inches long using standard measurement from tip of the nose to base of caudal fin.
He has been in an 80 gal tank from the outset ,and twice weekly 50 percent water changes with varied diet that includes .. frozen chopped krill, frozen brine shrimp,beef heart,spirulina shrimp,chopped earth worms,freezedried crickets,freezedried plankton,frozen bloodworms,and ocean nutrition's cichlid formula.
Electric Blue Dempsey's are said not to grow as fast, or as large as the Regular Dempsey and I would agree if mine is any indication.
Would give it frequent water changes,varied diet,and lot's of room if it were my fish. Not certain it will ever be able to hold it's own against an oscar that grows much more quickly in my expierience, and Oscars who claim a tank as their own, seldom tolerate new guests very well.
As a young boy I kept a regular dempsey and a red Oscar together from juveniles to adulthood and both appeared to get along ,even sharing the same cactus pot for resting area. Came home from school one day to discover that the dempsey had decided that the tank was no longer big enough for the two of them and had killed the Oscar, both fish were around eight maybe ten inches in length..
The Oscar you have, may always be considerably larger than the Electric Blue dempsey and may never accept it so a seperate larger tank for the dempsey may be something you might consider.