ob-gyn is not a very tough residency to secure. Recently i've seen alot of fmgs match in this field( although alot of these fmgs were actually americans who went to carribean schools). The thing i've noticed about ob especially is that these people really look more at personality and how you fit in than any other field since residents in this field really have to be able to work well with one another. otherwise if your a us grad with even low board scores you'll still match somewhere granted it won't be at a prestigious university program.