Calpundit pays tribute to the strong female characters in the Oz books. He also makes a (presumably half-joking) case that Ozma was a proto-transsexual:
The lovely Ozma of Oz, pictured on the right, is the kind, efficient, and obviously female ruler of Oz who takes over after the wizard is exposed. But who is Ozma?
Answer: Ozma is Tip, a boy throughout the entire second book, The Land Of Oz, who is transformed into a girl by Glinda when all the men prove themselves to be hopelessly muddled rulers.
But wait, you say, Ozma is no transsexual. This is magic, not reality, and magic is an entirely different kettle of fish with no social implications. Not so. In fact, Baum was a theosophist who believed that magic and science were just two sides of the same coin.
Now, I grant it's been a looong time since I read the book, but as I remember it Ozma was born female but transformed into a boy to disguise her identity. Glinda was thus restoring her natural state. If I were an aspiring transsexual, I would not take heart from this.
Eve Tushnet also asks a good question: "...I wonder how his case for Baum's feminism can be squared with the disastrous (if I'm recalling the book correctly) results of Jinjur's army of women?" Again, long time since I read it, but the Jinjur story certainly seemed like an attack on feminism. The all-female army briefly takes over the Emerald City and forces all the husbands and wives to switch roles, making women have jobs and men care for the children. This makes everyone miserable until the coup is overthrown.
This story is a bit strange, because as Calpundit says, Oz is basically a matriarchy. But one fact he left out is that all those strong female characters are unmarried and childless. Baum could envision women ruling whole countries but evidently he couldn't envision them in any other family role but the traditional one.