Rora - Flathead Mountain was ruled by the Three Adepts at Magic until their transformation into fishes by the neighboring Queen Coo-ee-oh of the Skeezers. This created a power vacuum that allowed a Flathead whose name is never revealed to set himself up as Supreme Dictator, or Su-Dic for short. He and his wife Rora then set up a policy of confiscating their enemies' brains. See, since Flatheads had literally flat heads, they carried their brains around in cans in their pockets, making them easy for corrupt dictators to steal. When the leaders obtained the extra brains, they were able to learn magic, the Su-Dic becoming a sorcerer and Rora a witch. Rora mixes up some fish poison to dispose of the transformed Adepts, but Coo-ee-oh, whose fate is tied to theirs, changes Rora into a golden pig. She's unable to do magic in this form, and keeps it until Glinda disenchants her.
Coo-ee-oh - As I'm sure you know by now, she's the Queen of the Skeezers, a group of Gillikins who live on an island in a lake. According to the Urban Dictionary, "skeezer" is a slang term for an ugly, low-class prostitute, but it presumably didn't have this meaning back when Baum wrote Glinda. But then, Peter Glassman of Books of Wonder argues that the Tin Woodman's line, "A good tart is far more admirable than a decayed intellect" (from Land) is using "tart" in its slang form, so maybe I shouldn't put it past Baum. Coo-ee-oh is described as "elaborately dressed as if she were going to a ball," but also "too thin and plain of feature to be pretty," apparently without realizing this herself. The Three Adepts taught her a good deal of magic, and she declared herself the world's only Krumbic Witch. Wicked uses the similar term "Kumbric Witch" to refer to some kind of ur-witch, but there's no indication as to what "Krumbic" might mean, if indeed it means anything. The Queen turned the Adepts into fishes, but they cursed her to shrivel up and lose her magic if any one of them died. With her magic, Coo-ee-oh is able to make her island rise and sink, and to command a fleet of submarines. (Oz was always pretty up-to-date in some respects, if rather quaint in others.) Rora's fish poison turns the Queen into a diamond swan, and Ozma and her friends make their way into her control room, where Dorothy correctly guesses that the syllables of Coo-ee-oh's own name are used to control the magical machinery. Just goes to show that people using really obvious security codes was a problem long before the advent of the Internet.
Faleero - The ancient and hideous Princess of Follensby Forest is described as both a fairy and a witch. If I may toot my own horn briefly, my own explanation makes her a fairy who studied forbidden witchcraft, and was banished for it. King Pompus of Pumperdink tries to marry his son Pompadore to Faleero in an act of desperation, but when Pompa finds his own princess, the old fairy is angry. She gets revenge by marrying King Pompus' brother Kettywig and then transforming the rest of the royal family, hence setting herself up as Queen of Pumperdink, and instituting a puritanical reign in which all fun is illegal. Fortunately for Pumperdink, the Red Jinn of Ev disenchants the other royals, and Ozma punishes Faleero for her wicked deeds. Faleero has equally repulsive three ladies-in-waiting who might also be fairies and/or witches, and while they're not named in the canon, Fred Otto's short story "The Invisible Fairy of Oz" calls them Audia, Claudia, and Fraudia.
The Imperial Squawmos - She's actually a Cookywitch, which is identified in the text of Cowardly Lion as "next in wizardry to a sorceress," but it seems to me more likely that a Cookywitch is simply a witch who specializes in cookery-related magic. This particular Cookywitch has preserved the inhabitants of a Munchkin town in jars and cans, and they believe that they'll spoil if exposed to the elements. The Squawmos is a tall woman who carries around a giant iron fork and spoon, which she sometimes uses to smash things.
Wunchie - Identified in Handy Mandy as "a Witch of the West," but since Ruth Plumly Thompson got directions mixed up in her later books, she (Wunchie, not Thompson) is actually a Witch of the East. According to her former servant Himself the Elf, she lived in the Munchkin Mountains for 1000 years, and controlled as many kings in the region as she could. Himself was summoned by a silver hammer, and could do just about anything. She was eventually destroyed, not by water or a falling house, but by eating explosive jumping beans given to her by the Wizard of Wutz. That's a pretty sad way to end a millennium of being the power behind multiple thrones, isn't it?
The Witch of the Woods - Before being invited by the publishers to start writing Oz books, Thompson had written a few stories about Pumperdink. In her second Oz book, Kabumpo, she placed this kingdom in Oz. It's never entirely clear whether this is the same Pumperdink, but I see no reason why it wouldn't be. Sure, the royal family is different, but maybe these stories took place during the reign of an earlier king. One of them, "The Wizard and His Purple Beard," is about...well, a wizard with a purple beard (duh), which gives him away whenever he tries to pull tricks on the Pumperdinkians. He tries to get the Witch of the Woods (presumably not the same as Faleero, although she also lives in a forest near Pumperdink) to make his beard vanish, which she does, but she makes the rest of him vanish with it.
Painted Witches - In Lucky Bucky, the Wizard of Oz makes magic paint for the people of the Emerald City to paint the palace walls. It ends up being TOO magical, though, and the paintings come to life. One of them is the painted Mombi I mentioned on Sunday, but three other witches to escape from pictures are Aunt Geranium, Little Blue Schoola, and Plush. They're presumably witches that the Ozites had dealt with in flesh-and-blood form at some point, but we never learn the details. We do know, however, that Aunt Geranium is invisible whenever a bird sings in the area, and she makes geranium buds appear on people's noses. Schoola's main thing is breaking shoestrings, while Plush is apparently more powerful and threatening than her ridiculous compatriots. Lucky Bucky also mentions two other non-painted witches, Curly Ah-Ha-Do and the Thimble Witch, but we really don't find out anything about them other than that the latter used to ride a broom named Po, until she broke her (yeah, Po is identified as a female broom, although I have no idea how you can tell) over a farmer's head and left her on the roadside.
Fanny - The Weather Witch lives on top of the tallest mountain in Oz, where she controls the weather from a windmill that makes wind, rather than harnessing it. In appearance and personality, she's a far cry from either the withered old crone or the young pin-up girl, instead being a tough Rosy the Riveter type who wears overalls and carries a giant wrench. One reader said she was the most blatant lesbian to appear in an Oz book.
Magda - While I'm mostly sticking to canonical and quasi-canonical works (although I mention a few other stories when these witches reappear in them), I've already mentioned a few of Eric Shanower's characters, so I might as well add another. Besides, if Dick Martin's Ozmapolitan is generally considered pseudo-canonical, why not Shanower's books? Magda is an old hermit who appears in Shanower's Giant Garden, who has invented a way to make vegetables grow to large proportions. Unfortunately, it works too well, and almost destroys Uncle Henry and Aunt Em's new farm. It also makes Dorothy grow to enormous size, and I can't actually remember how everything gets sorted out, although I know it does. Maybe I need to re-read that book.
I'm not including the two Yookoohoos, Mrs. Yoop and Reera the Red, as I consider a Yookoohoo to be a separate kind of magic-worker from a witch. There is, however, one more post I'd like to make on this subject. It's about witches who appear in the Oz series or related books, but don't live in Oz proper, including that famous Ixian vixen Queen Zixi.