Princess Ozma of Oz is a trans character?

Well, yes she is! She first appears in L. Frank Baum's The Marvelous Land of Oz (1904), which was Baum's second Oz book and she appears in everything after that.

It is well known on social media (Reddit, TikTok, Tumblr, etc.) and the trans community and even some of the Oz fan sites that Ozma is a trans and queer icon, no matter what Baum intended. But, you wouldn't know it by looking at mainstream media (I found zero articles) or even Wikipedia, where Oz writers choose to have a very limited idea about what trans or queer mean and so ignore them altogether.

For backstory, Ozma was born female but, after some magical transitioning, "he" grows up in a male body (named Tip) and thinks he is a boy. "She" later transitions back to her female identity, albeit reluctantly:

"Why, I'm no Princess Ozma — I'm not a girl!"
 "You are not a girl just now" said she, gently, "because Mombi transformed you into a boy. But you were born a girl, and also a Princess; so you must resume your proper form, that you may become Queen of the Emerald City." 

Transitioning is effected via a magic potion, herbs, powders and the word "Yeowa!" shouted in a very loud voice, but who am I to judge the method. Ozma, it turns out, is also a fairy, but that word leads to even more gender confusion. Furthermore, she appears to have a very close relationship with Dorothy, who returns to Oz and they hit it off. As lesbians? There are plenty of examples from Baum's novels of how much they adore each other and want to live together, in order to support this argument. In Baum's books, gender itself is magical.

Traditionalists rightly point out that things were different back then, but what should we make of that illustration at the top? That's the sorceress Glinda in a rapturous embrace with Ozma. Directly under that illustration we encounter these lines:

"You are my prisoner, and it is useless for you to struggle any longer," said Glinda, in her soft, sweet voice. "Lie still a moment, and rest yourself, and then I will carry you back to my tent."

Hmmmm. Glinda is actually speaking here to the sorceress Mombi, not Ozma, but the juxtaposition seems intentional. If W.W. Denslow illustrated the Chinese themes in The Wonderful Wizard of Oz, the new illustrator, John R. Neill outdid himself with sexually ambiguous poses like those shown here. Baum also explored gender ambiguity in other books like John Dough and the Cherub (1906) - which nobody reads - but it's Ozma as a queer trans woman that is back in vogue and that's to Baum's credit as a writer.