something you need to know about advertisements for nurses in any of the papers. . .when you see those, the places are usually pretty desperate for nurses. most nurses who are in the profession don't even bother looking at those ads. the reason is because we know that we only have to walk into a hospital or pick up the phone and call the nurse recruiter at any hospital to find out what their job openings are.
when you graduate from nursing school you will be in the position of looking for your first job. it's a once in a life occurrence. the nurse recruiters from hospitals know this. when you go looking for that first job, you just specify that you want to work in l&d. if a hospital is interested in taking on a new grad in that department and putting in the time to train you, they'll hire you. there are a lot of hospitals that do that. they don't advertise it in those papers. in fact, look through those papers again and you probably won't see any ads mentioning that new grads are welcome. however, trust me, they all take on plenty of new grads.
to answer your specific question about there being special classes you should be taking to get into the ob department after the graduation, the answer is no. there are no such classes. what happens is you do as i mentioned above. or, in some cases, some hospitals will tell you that they would prefer that you have at least one year of experience working on a general medical unit first before they will consider taking you on an ob unit. the ob/l&d units give you all the on the job training you will need to work there. you have to finish nursing school and get your nursing license first though.