god damn! who will be the first to die? Both types of fish are jumpers, and when attacked, can and will jump out of an uncovered tank. The dempseys in particular will go for the sides of the oscars, and each other, screaping off a lot of scales, which can lead to infection. It's likely one fish will begin systematically killing off the others over the next few weeks.
I have extensive knowledge of these two species. You have a bad combo going there in a tiny tank. 55 gallons is nothing. I've kept aquariums my whole life and actually worked at that aquarium at the local zoo. hit me up if you need advice and... good luck...
