I don't know how to get Svorak on OS X, but I did want to point out... you get the characters you mentioned in the same way in Dvorak as you do in the standard US keyboard config, of which you may not be aware:
opt-a -> å
opt-u + a -> ä
opt-u + o -> ö
The way the US keyboard works is that each special diacritical mark is tied to one of the characters most commonly seen with it. So umlaut is on the u, the "hat" is on the i (î), the two accents are on the e and the (exception) tilde/forward-single-quote key keys, the tilde is on the n, and so on.
That might get you started while you look for Svorak.