There are two separate diseases in the hobby referred to as HITH. One is contagious, the other isn't. If you do a search here at AC, you will be able to find numerous threads that discuss this issue.
In the meantime, there are several things you can do. The first is to increase your water changes and possible add a little crushed coral (1 tsp or so) to the tank to increase its buffering capacity. That generally doesn't hurt in these situations. Because both of your tanks are overcrowded, you should be changing 50% of the water per week at a minimum. I would also take the Oscar in the 20 gallon tank back to your favorite LFS, because it will not do well in such a small tank and the ones in the 55 are already overcrowded (even just one oscar in that tank would be pushing it, in fact).
In the 55, since both fish are already affected and are having relationship problems to boot, you should try buying some "egg crate" divider material (available at any hardware store for about $5 per sheet), and make a divider for your tank. The divider will let water pass back and forth easily, and you can hold it in place with suction cup/bracket heater holders that they sell for units like Ebo Jaegers as replacements.
With greater water changes and coral buffering, and reduced stress from separation, your oscars should recover if they have the less serious version of HITH. The scars will then eventually heal over.
Good luck.