You might have a few months to watch them together, or the Oscar ,being smart like a dern velociraptor might chase it one day and notices It loses a scale. From that point forward,not only is it fun for the Oscar to chase it because it cannot get away,it also makes food he notices when he chases it( i.e. scales) . From that second on the Oscar is being rewarded twice over with fun and food. Eventually leading to it jumping out of the tank or being beaten to death because the fish isn't losing anymore scales for the Oscar to eat. Fish cannot reason. I hope I am completely wrong and they live great together. I being a human,can reason. That's the reason I posted my response. Seriously hope everything goes great. Just keep that in the back of your mind. If you notice the Chalceus acting skittish or you see the Oscar or the Dempsey make an aggressive move towards it,you better act. Just one night or day you're at work, if that long, is all it takes. Thanks