A 55 is too small for 2 oscars even if they pair up that will have issues from time to time. Oscars like to have their space. Not to mention the bioload of 2 full grown oscars in a 55 is way too much. Your Nitrates will be through the roof.
For 1 Oscar a 55 gal is bare minimum with a 75 really being a much better choice. For 2 you would need no less than a 110 gal tank.
A better choice for a 55 would be a severum, you could even get away with 2 of them, but that would be pushing it a bit.