General Thoughts: The OB/GYN shelf is generally considered to be relatively easier compared to some of the others (e.g. surgery), but don't let that be an excuse for slacking off - OB/GYN contains a lot of domain-specific topics that you might not have covered or heard of before, e.g. fetal heart tracings, chorioamnionitis, etc. What OB/GYN lacks in breadth, it makes up for it with depth.
UWorld: Again, UWorld for Step 2 CK will be a high-yield resource. For OB/GYN, there are 276 questions as of this writing. If you're in a 4-5 week OB/GYN rotation, start early with ~20 questions per day and you should be able to knock them all out in about two weeks, with the remaining time left over for a second pass and to review your incorrects. I found UWorld's coverage to be fairly well-balanced in terms of concepts covered.
Case Files Obstetrics and Gynecology: This handy book contains 60 cases, evenly split between obstetrics and gynecology. Each case covers a specific diagnosis/scenario and is followed by a few questions related to the topic. The cases are fairly succinct and therefore not comprehensive, but make for quick, high-yield reading and question review when you have downtime on the wards.
APGO uWISE: The APGO uWISE question bank is an excellent adjunct to UWorld. It contains 50+ topics, each of which contains 10 questions (with answer explanations), and is great for extra review and question practice. The only issue is that isn't free. Many medical schools will subscribe, in which case you should have received a login/registration link. You can also pay $175 for a 3-month individual subscription, but it's probably not worth it.
Obstetrics and Gynecology: The Beckmann text is often required/recommended by medical school curriculum directors as a text for the OB/GYN rotation. At 600+ pages, it's definitely not light reading and it's definitely not going to fit in the pocket of your white coat, but if you feel like you need a comprehensive resource or that UWorld +