I like melafix, seems to work pretty good. The salt with help some, but keep in mind that oscars' natural habitat has soft acidic water (read: very little salt).
Whoever told you that 2 adult oscars is fine for a 55 gal is crazy. A 55gal tank doesn't have the width needed for a fish that can get that large. I tell customers at work to think about it like this: It would be like yourself living in a closet that's long, yet too narrow to turn around comfortably in. A 75gal is the least that I recommend for a single oscar, let alone two. If it were a breeding pair, it's possible that a 75 would be ok. My oscar went nuts on a daily basis when she was 12in long and in a 55gal. Bought a 75gal and she's calmed down quite a bit. She's up to 14in now.