The Wizard's backstory was ret-conned and white-washed in the later books, but he was unambiguously a villain in Marvelous Land of Oz. Baum has him straight-up give baby Ozma to Mombi to hide so that he doesn't get ousted as the ruler of Oz.

Though, I should note that calling him "the ruler of Oz" is debatable considering the cardinal witches still ruled the four quadrants of Oz, leaving him with only Emerald City in the middle.

My pairing of Oscar Diggs and Mombi might seem like a bit of a crack ship at first glance, but it makes a lot of sense when you think about it. What motivation does Mombi have to hide Ozma for Oscar? I doubt a trade of some humbug magic tricks would be enough to convince her. It's not too big of a stretch to imagine them as about the same age, either.

Of course, you might find out I've gone in a slightly different direction for my version.