The name for Oz in "The Wizard of Oz" was thought up when the creator, Frank Baum, looked at his filing cabinet and saw A-N, and O-Z, hence "Oz." Hmmm...
You can take Baum's word for it, or you can take his wive's. Your choice.
We don't have to rely upon whatever Baum's children may have been told for verification, however, as Baum himself had offered essentially the same story many years earlier in a press release drafted to announce the reissue of The Wonderful Wizard of Oz in 1903:
This evidence wouldn't seem to leave much room for doubt, as Baum himself is undeniably the one person who knows how he came to choose the name, and this explanation comes straight from the horse's pen, so to speak. Baum's version does differ from the one offered by his son in that the latter places him in a roomful of children rather than alone in his study, but that difference might be dismissed as a mere literary embellishment on his son's part. Even Baum's version contains its own discrepancies, though, as various pre-publication references and copyright registrations reveal that Baum considered several titles for his book using the word "Oz" but not the word "Wizard" (e.g., "The City of the Great Oz," "The Fairyland of Oz," "The Land of Oz"), so clearly he had not "settled on the 'Wizard' as part of it" before coming up with the name 'Oz.'2 Moreover, Baum's wife Maud wrote to a friend in 1943 that:I have a little cabinet letter file on my desk that is just in front of me. I was thinking and wondering about a title for the story, and had settled on the "Wizard" as part of it. My gaze was caught by the gilt letters on the three drawers of the cabinet. The first was A-G; the next drawer was labeled H-N; and on the last were the letters O-Z. And "Oz" it at once became.
snopes.comThe word Oz came out of Mr. Baum's mind, just as did his queer characters. No one or anything suggested the word — or any person. This is a fact.