There's no way to tell. The Air Force will never release information regarding the competitiveness of a specialty. I will say that 17 people matched into ob/gyn today, but again, there is no real useful information to be taken from that number.
I see that you're a pre-med. Realize that your interests will more than likely change over the next few years.
It depends on what you mean by competitive. It is definitely more competitive than the civilian world where you are pretty much guaranteed to match somewhere no matter what your grades/board scores are. Typically there are about 30 people applying for about 14 slots (6 military/8 civilian). Usually the military keeps the best candidates for itself, the mediocre people get civilian deferment and the poor candidates do not match at all into ob/gyn.
Thanks for the feedback - I'm already accepted to a school, and for that interview there, I actually wrote a 10-page paper with research statistics and references about some nonprofit work I hope to do in OB/GYN one day with certain populations. I'm pretty certain this is what I want to do, and I've done a lot of shadowing to make sure also. Those stats are fairly decent. I plan to work hard in medical school, so hopefully I will get a spot somewhere.