If Oz was a D&D campaign and guessing the classes off the show are two different things. I'm with those who say outside of the witches, you'd have lots of NPC classes for everyone. My picks for their classes are as if it was a D&D game using core classes.
* The Witches: Sorceress
* The Wizard: Rogue, with 'bluff', 'disguise', and 'use magic device' max out to pretend to be a wizard
* Order of the Ruby Slippers:
- Dorothy: Cleric - she healed the warforge Tin-Man, casted break enchantments on the charmed Lion, and so on...
- Scarecrow: Wizard - unfortunately his INT wasn't too high so he casted only cantrips...
- Tin-Man: Ranger
- Cowardly Lion: Barbarian
* Monsters:
- Munchkins - halflings
- Flying Monkeys - winged goblins