Just as Middle-Earth is a European fantasy land, Oz is an American one. That said, does its geography have any basis in that of the United States? I would say there’s some. In The Wonderful Wizard of Oz, the eastern Munchkin Country is mostly civilized and has a lot of forests, while the western Winkie Country is wilder and largely made up of plains. As L. Frank Baum lived in a time when the American West was being settled, and he himself lived in several different parts of the nation, that’s unlikely to be a coincidence. I don’t know that this carried over so much into the southern Quadling and northern Gillikin Countries, however. The main trait of the Gillikin Country as it developed is that it’s the most dangerous of the four. While Baum appears to have always considered the countries to be symmetrical, the actual details developed over time. In his last few books, however, he seemed to have a pretty good idea as to which parts of Oz were explored and which were not. Ruth Plumly Thompson was a bit more haphazard than Baum in her placements of previously undiscovered communities and features, sometimes placing them in areas with which the main characters should probably be familiar by the time of her books, but I hardly think she proceeded with no rhyme or reason. Her Oz was rather more European than Baum’s in many ways, however. While Baum distributed a few largely autonomous kingdoms throughout the land, Thompson’s books were inundated with them. Not a problem, just a different take on the same fairyland, and perhaps a necessary one for Thompson’s style. As far the placement of her kingdoms goes, I’ve seen it suggested that Ragbad in Grampa in Oz, a Quadling kingdom that grew and sold dress goods, had similarities to a Southern plantation. Hence, she might well have returned to the influence from American geography of the first book. And while the old-fashioned and proud kingdoms of Pumperdink and Regalia being in the dangerous Gillikin Country might not make sense at first glance, I get the impression that it had to do with purple being the traditional royal color. Her Munchkin Country also remained largely forested, just as Baum had left it.
When it comes to Ozian geography, a difficult topic is that of where Oz is supposed to be located. As I mentioned here, the first two books suggest that Oz might actually be on the North American continent, separated from the civilized part merely by the desert. In fact, Alexander Volkov’s Magic Land books retain that idea throughout. Baum, on the other hand, started placing other fairylands across the desert from Oz as early as Ozma. Since Dorothy washes ashore in Ev while on the way to Australia, the Ozian continent (such as it is; I don’t think it’s really big enough to be a continent by our standards) might be in the Pacific Ocean. I don’t know of anything in Baum that specifically contradicts this location, but there are some references that make it a bit difficult. For instance, how would a tornado have blown Dorothy’s house to the middle of the ocean? Baum also placed Merryland from Dot and Tot of Merryland on his map of the lands surrounding Oz, and Dot and Tot accessed that country from a river in New York. Outside Baum, Pirates has Peter Brown ended up in the Nonestic Ocean after being blown off a boat at Cape Hatteras, North Carolina. The fact that the Nonestic Ocean has its own name is also an indication that it is probably separate from the Pacific, as is the discovery of a whole new continent in Captain Salt.
Perhaps partially inspired by Tolkien’s idea that the Valar changed the shape of the world to keep mortals from reaching the Undying Lands, it may be possible that the location of the Nonestic world has changed over time. Perhaps once it WAS located in the Pacific, but the fairies moved it once mortals were likely to come across it. Indeed, one idea I’ve seen before is that Glinda’s enchantment to make Oz invisible to outsiders at the end of Emerald City affected not only Oz but its neighbors as well. There’s a pretty good short story in an old Oziana, “A Side View of the Nonestic Islands,” that utilizes this idea. Another Oziana piece that might be sort of relevant is George Van Buren’s “Zimbo and the Magic Amulet,” in which the Pillars of Hercules are identified as a gateway between our world and the fairy one. Then there’s a presence of King Anko and Aquareine’s palace on James E. Haff and Dick Martin’s map, even though The Sea Fairies strongly implies that they are in the Pacific. And getting back to Middle-Earth, I seem to recall someone on an online Oz forum saying it might have been Oz to which the Elves and Bilbo and Frodo Baggins eventually sailed. It doesn’t look like it, but a lot of things can change in 6000 years.