OP, I'm not trying to be rude, but you never answered the original question. Without the tank size, it's like asking a chemistry science question without all of the variables. A person could have an explosion on their hands. It doesn't matter which fish will get along better with your dempsey if you're cramming them in a 20 gallon, the tank is already too small for your JD by himself once he hits maturity. I think most everyone would agree the oscar would be the better pick between the two but unless you have a 75 gallon tank or larger to put them in a couple months, it's not even worth it to try. So how big is the tank? We can't do your situation justice without that info.