Hi! Just wanted to add a bit to allthingsbright's response. That is correct, you will get your RN through your choice of ADN/BSN routes and it will be generalized. Then, when you are hired onto an OB floor, they typically will pair you with someone who will "precept" you for a certain amount of time on an orientation. At that point, you will learn many more skills specific to the area you are going to pursue, which in your case is OB. They will teach you far more in-depth detail regarding your specialty and most hospitals have orientations that are set up specifically for "new grads".
Best of luck in your pursuit! Enjoy the journey!