If you assume that Pastoria had a wife, did Lurline make her carry a baby, similar to how the virgin Mary gave birth to Jesus? But the Famous Forty Oz books never mention Pastoria's wife. (Mr. Flint in Oz by Ray Powell introduces Ozette, Ozma's mother, but this is not a Famous Forty book. Nor do I really consider it canon.) From the indications in the Famous Forty, Pastoria was a single father, Lurline providing him with a daughter that he adopted.
In The Marvelous Land of Oz, it is said that the Wizard turned Ozma over to Mombi, but Ozma seems to indicate in Dorothy and the Wizard in Oz that Mombi was responsible for her abduction as a baby. In the story Oz and the Three Witches by Hugh Pendexter III, it explains how the Wizard turned Ozma over to Mombi to protect her. That is my general idea as well.
Finally, Tip decided to scare Mombi one day by building Jack Pumpkinhead. While Mombi was startled, she brought Jack to life. No longer needing Tip's services, she decided to turn him into a marble statue. Tip decided to run away with Jack, taking with him the Powder of Life. Thus, he had the adventures in The Marvelous Land of Oz: how he brought the Sawhorse to life, allied himself with the Scarecrow and Tin Woodman against General Jinjur's revolt, met the Woggle-Bug, helped create the flying Gump, and met Glinda who captured Mombi and forced to reveal his true identity.
Tip was hesitant to become a girl, but he eventually agreed to be restored to his true form. Mombi performed the spell to restore Ozma, and soon, she was the princess ruling the Emerald City.
Ozma's transition from Tip is not a major theme in the Famous Forty. Some excellent stories outside the Famous Forty have addressed it, but they all seem to note that the transition was rather smooth. As she herself says, I'm just the same Tip, you know; only—only—" Jack Pumpkinhead concludes this by saying "
Only you're different!" Ozma apparently embraces her full identity.
In Ozma of Oz, Ozma sets out to rescue the Royal Family of Ev from the Nome King, meeting Dorothy for the first time, the two becoming fast friends. But however noble Ozma's intentions were, her actual right to get involved and confiscate the Nome King's magic belt has been questioned. Not to mention that she is quite unprepared to meet the Nome King.
In Dorothy and the Wizard in Oz, Ozma welcomes the old ruler of Oz to remain as a permanent citizen and important member of her court. The Road to Oz shows Ozma throwing a grand birthday party, and The Emerald City of Oz features Ozma welcoming Dorothy and her family to Oz to stay for good. However, it also sees Ozma deciding not to defend her fairyland, deciding to reason with invaders set on conquering Oz. She says that no one should have the right to kill another, and she does have a point, but likewise, they do not have a right to invade Oz. (Except that Roquat wants his belt back.) It is the Scarecrow who comes up with a non-violent way to defeat the invaders.
In later Oz books, Ozma generally rules Oz, making several questionable decisions. Seemingly, it was she who abolished currency in Oz and set up a somewhat communist format in which everyone helps each other out by sharing their goods with each other. (Although we frequently meet people in Oz who prefer to keep to themselves.)
Baum allows Ozma to be abducted once in the Oz series in The Lost Princess of Oz. Thompson occasionally had the Emerald City nearly conquered or invaded, but Ozma was really only abducted a couple times, though one of those times involved her being put under an enchantment and almost everyone in Oz forgot about her. (So I guess the Emerald City actually was conquered, but only Dorothy and Pigasus knew it.) While Ozma might not be the wisest ruler and has a lot to learn, she seems very capable. At least she knows when to seek help (usually from Glinda). She may not be a perfect ruler, but she's getting there.
Due to Ozma's seemingly smooth transition from Tip to her true self, Ozma has been viewed as a transgender icon. Some people have even cited Ozma's story as inspiration for their own coming into their own gender identity. In addition, some interpret her relationship with Dorothy as more than just close friends, and see her as a lesbian character as well.
So, Ozma, a girl, rules Oz. Now what? Talking chickens?