In The Magic of Oz, the Wizard learns the magic word "pyrzqxgl" which can transform anything or anyone into anything. By the end of the story, the Wizard is presumably the only one in Oz who knows it, except maybe Bini Aru. The only other people who knew it had their memories wiped.
Consider that: a word capable of complete transformation. And only one person knows it.
Did he tell Glinda? Well, Glinda would know he had the secret by the Book of Records, unless he could alter it with the word.
Did the Wizard ever use the word after The Magic of Oz? We aren't told, though there are a variety of places in the Famous Forty where he could have. But where's the fun in that?
Actually, there's one point in the Famous Forty where I feel sure the Wizard did use "pyrzqxgl." It's in The Giant Horse of Oz as Ozma and company set everything back to rights in the Ozure Islands. The Sea Horses are long dead due to a monster eating them and only their bones remain. But after the monster's destruction, the Sea Horses appear to have come back to life. Thompson's explanation:
...when they all crowded curiously around the Wizard he merely shook his head and muttered that restoring a herd of sea horses from a pile of bones was quite easy—if you just knew how. And with this answer they were forced to be satisfied.You see that?
THE WIZARD BROUGHT THE DEAD BACK TO LIFE.
What else can he do? Create life? Life that answers only to him? Slim down? Fatten up? Look at the continuity in John R. Neill's pictures of him! His weight (and age) is all over!
Actually, in Wonder City, doesn't he make himself unrecognizable to Number Nine and turn Jenny Jump into a younger version of herself and remove parts of her personality? This is one great and terrible guy we're dealing with!
So, does Ozma keep Glinda and the Wizard around because she's scared of them, or because she needs to keep an eye on them?
This has been a tongue-in-cheek but completely plausible speculation blog entry.