No, they really don't need salt. Oscars, IME, respond very well to treatments. Also, they are hardy fish IMO, so keeping their water clean will, for 90% of the time, keep them disease free. I never used salt when I used to have oscars, infact I used to own them when I knew very little about fish keeping with very few problems... the occasional ich breakout.
What size tank are you housing them in currently?