Working part time at a hospital will get your foot in the door and if you are a good employee (dependable, friendly, hard worker) you will probably get extra points when they decide to hire the recent new grads. Lots of student nurses work as secretaries on the nursing units. This is a good way to learn the routine of the unit, common lab and other orders, how to work the computers, etc. Later once you are in your actual nursing classes they might have positions specifically for nursing students, similar to nursing assistants the first year and then with some expanded duties the second year.
If you are fully certain OB is the area you want to be in, try to aim for that area (labor & delivery, antenatal floor, postpartum, well baby nursery, mother-baby unit, special care nursery, NICU, etc). If you end up on med-surg that is not a minus because you will realize that all kinds of women have babies and many of them have conditions which are treated in a med-surg area (asthma flare ups, heart conditions, strokes, sickle-cell, kidney stones, gallbladder problems, etc.) so you can learn a lot and be prepared for when later as an OB nurse a mother comes in pregnant and in distress for something unrelated to her pregnancy.
I always knew I wanted to work with babies and I asked my OB instructor if it was truly necessary for me to work 2 years of med-surg first (I had worked med-surg for on year as a student nurse every Friday & Saturday night for the last year of my nursing education) and she said that if I knew for certain in my heart that I wanted to work with the little ones that she would rather see me go straight to the nursery rather than force myself to do a 2 year med-surg stint and end up leaving the profession because I was unhappy. I started in a well baby nursery, then did mother-baby (learned postpartum moms & some stable undelivered moms). I appreciate her advice and am happy with my decision. My only drawback is that I realize that today (20 years later) I know very little about adults. I still subscribe to two general nursing journals along with all my neonatal ones but I would feel lost on a med-surg floor or any floor except postpartum at this time in my career. Keep that in mind as you make your decisions. Any experience is good experience and sometimes life puts you in places you don't think you want to be but in hindsight you are ever so glad you were put there because of the learning experience it gave you.
Enjoy your education, study hard, remember the basics are incredibly important as everything else builds on them (A&P, Micro, etc) and remember that nursing school is intense but you will make it through just like so many of the rest of us did. Good luck in your schooling and career! One last thought, go ahead and join AWHONN the main nursing organization for OB nurses. You will probably get the student's rate to join and will get journals and loads of other information access concerning OB nursing. It would be good side reading to get your feet wet, but don't neglect your nursing studies. http://www.awhonn.org