It's a specialty certification, and you can have it in any number of subspecialties within OB. Low-risk Neonates, Maternal-Newborn, Inpatient OB, and High-Risk OB are the ones I can think of off the top of my head.
You have your specialty certification in any one of the above, but you just have RNC behind your name. Some people specify RNC-OB etc. but others don't. Hope this helps.
You can have an RNC certification in lots of specialties other than OB too. I had one in psych awhile back. Nowadays RNC is the certification acronym for some who got their certification awhile back (if ANCC), but ANCC changed it to RN, BC (board certified) not too long ago.
RNC is a certification that results from taking a terrible, awful, mind-stressing test after you have a certain amount of experience in your specialty (I'm kind of kidding about how bad it is, but it did remind me of the NCLEX). There are a few places that offer it but ANCC (American Nurses' Credentialing Center, I think) is one of the biggest. You sign up for it, pay your money, turn in a bunch of verification that you're qualified to take the test, and then take it. Then you get the C or BC to put after your RN, along with a certificate and a nifty little pin. That's how it worked for me anyway.
I believe the testing organization will allow your facility to be a testing site (paper and pencil) if you have a certain # (15, I think?) of applicants. My hospital did that this year for the OB nurses.
ANCC does not offer OB certification, only NCC. The credential is now RNC-OB, RNC-MNN, RNC-LRN, or RNC-NIC. I use the RNC-OB on "official" documents (applications, professional correspondence, etc..), but only RNC in my every day charting.
I personally thought the test was a lot harder than NCLEX - a lot of physiology questions. I am glad I took it though. Studying for the test made me look hard at evidence-based practices. I'm sure I have annoyed people at work with my "Well, AWHONN says" comments!