There’s a fair amount of magic in the Oz series based on illusion, although exactly what that indicates is a little ambiguous. An illusion is a distortion of sensory input, different from a hallucination in that that actually is a stimulus, just not what the mind thinks it is. Sleight-of-hand tricks like the Wizard of Oz does with the Nine Tiny Piglets are considered illusions, but I’m thinking more of illusions produced by actual magic.
The Marvelous Land of Oz has Mombi using her witchcraft to produce several illusions in an attempt to stop the protagonists from returning to the Emerald City. The first is a large field of whirling sunflowers that sometimes took on the features of Jinjur’s Army of Revolt. The characters manage to make it through this one by running through quickly, with the ones who are able to close their eyes doing so. The Queen of the Field-Mice then leads them through a river, a granite wall, a bunch of roads, and a sheet of flame, none of which are real. All can be conquered simply by moving straight through them, but for some reason the mouse seems to be able to see through them more easily than the others. Come to think of it, she also wasn’t affected by the poppies.
In Lost Princess, the wall surrounding Thi turns out to be an illusion, as does an army that Ugu conjures.
Shadow Mountain in Lost King also looks quite solid, but turns out not to be a physical object at all. In pretty much all of these cases, the illusions are essentially three-dimensional projections, as everyone can see them in realistic detail, but there’s no substance to them. The illusory wall and gate in Patchwork Girl apparently works a little differently, at least according to the Shaggy Man, who says it’s only there when you can see it.
In the other cases, there’s no real object there at all, regardless of whether it’s visible. We also occasionally come across invisible walls, basically the opposite of the walls that can be seen but aren’t really there. There’s one in the base of Flathead Mountain, and one surrounding Nimmie Amee’s house at Mount Munch. In the latter case, it’s said to be made of solid air, created by magic left behind by the Wicked Witch of the East.
Interestingly, Stratovania is also said to be constructed of solid air. In both cases, the magic must enable air to solidify at a temperature in which people can survive. There’s dry water in the Rubber Country of Ev, and I suspect that might also be what the Tin Woodman uses in his pond of tin fishes. The most confusing case of illusion might be on Mount Phantastico in Emerald City, where we’re told that Guph sees the Phanfasms as grotesque beings living in rock huts, but they’re actually dressed in fancy clothes and have “a brilliant and gorgeous city.”
It’s not entirely clear whether what Guph saw was projected on top on what was really there, or the Phanfasms affected Guph’s mind. For that matter, are they actual shapeshifters, or is that all deception? And how does it benefit them to not show the Nome their real city? I’m not sure we’re meant to know, although Edward Einhorn’s Living House does indicate that they can physically shift forms. Queen Zixi is another interesting case, as it’s said that she sees her true form, that of an old hag, when she looks into a mirror, while everyone else sees a beautiful young woman. She is, however, quite active for an ancient woman. That could just be due to her magic, though; we have the precedent of the Herkus being incredibly physically strong despite appearing frail, and at least in the Thompson books Mombi is surprisingly physically fit even though she uses a walking stick. She “seemed positively tireless” to Snip, and Herby reports that she “had the strength of ten men.”
There is certainly magic in Oz that can directly affect minds, causing forgetfulness, changing moods, reading thoughts, and even creating brains. In the very first book, the Wizard relies heavily on the power of suggestion, not working any magic but convincing the Scarecrow, Tin Woodman, and Cowardly Lion that he has. The Scarecrow and Tin Woodman are somehow able to hypnotize the Guardian of the Magic Muffin Tree simply by twirling an axe and speaking.
The straw man refers to it as “a device often used by sorcerers and magicians.” What we don’t typically see is the sort of magic described in the Discworld books where people can be made to think they’ve been turned into frogs, but there’s no real transformation. Transformations in the Oz books can be real or illusory, but they aren’t simply psychosomatic. I guess you could say there are a lot of illusions, but not so many delusions; I can’t think of an occasion where an experience was simply in someone’s head, although I suppose some scenes can be interpreted that way by readers. And then MGM came along and made the whole place a fever dream.