Remember the Wicked Witch of the West’s crystal ball from The Wizard of Oz? Like when Dorothy sees Aunt Em in it, only for her to be replaced with the Witch herself? As anyone who’s read the book can tell you, L. Frank Baum’s original was rather weirder. His Witch had one eye that was as powerful as a telescope, and as such didn’t need a device for long-distance viewing. While the crystal ball in Wizard displays actual footage of what the Witch wants to see, it seems that people who use such instruments in real life tend to claim they see much simpler images in them, or that the ball is used to put them into a trance. Of course, it’s largely just a prop, as seen with Professor Marvel’s crystal earlier in the film.
I can’t actually recall any crystal balls in the L. Frank Baum books offhand. Long-distance seeing tended to be accomplished by more straightforward sorts of magic, like Ozma’s Magic Picture. Mombi uses a magic mirror to predict trouble in The Marvelous Land of Oz (how it works is never specified), and Glinda has such a mirror in Glinda of Oz, which she uses to find Button-Bright in the forest. Then there’s the King of Bear Center, whose magic wand can produce realistic-looking images of whatever he wants to see.
Crystal balls do appear in several of Ruth Plumly Thompson’s Oz books. Wumbo the Wonder Worker in Gnome King has “a great silver crystal ball on his desk,” which apparently tells him that Ruggedo is being dishonest.
Gloma in Wishing Horse has a black crystal ball that she uses to determine the nature of Skamperoo’s magic, and another one is used to transport various sorts of dangerous magic in an attempt to destroy Dorothy and Pigasus.
Regalia has an amethyst ball that flashes when a prospective king passes one of his tests. Quite appropriately, Crystal City in Ojo has “a huge crystal ball” that belongs to the court sage. It warns Ojo of danger and tells Realbad how to break the spell on the town, both in flashing text.
In Yankee, Jinnicky uses a crystal ball for various purposes, but I don’t recall viewing being one of them. And beyond Thompson, there’s the Oracle in the McGraws’ Merry Go Round, which is a crystal ball that resembles a snow globe.