FWIW, a quick head-count in my (9-to-5) office gives 2 women out of 10 people. Going up one level to the whole department, there's a couple dozen more men and only two more women, one of which is the secretary.
Looking back to previous employments, that's a pretty normal ratio. In my previous job there was one women in a team of eight. In the job before that, there were
no women among the development staff (but three secretaries and two women in HR).
Even at demo parties, a scene about as relaxed and diverse as you can get and still be about computers, there were distinctly less than 50% females in attendance.
So OS Dev simply reflects the computer / programmer scene as a whole.
Moreover, there is not even a
setting in the user profile to select male / female / other / won't tell. And there is no reason for a woman to say "by the way, I am female" in a casual discussion about CPU registers or IPC protocols, just as none of the guys run around flaunting their masculinity when talking about compiler settings. So how would you know how many women
are reading this right now?
And...
...why would it matter?
From my experience, both previous and in my current team, software is a field where, more than anywhere else, people tend to simply ignore such trivialities as to whether you carry your reproductive organs on the inside or the outside. You are what you know. And, at the end of the day, that is how it
should be, shouldn't it?