Experience the real exam by having a mock oral session simultaneously with the 2 examiners. Nothing provides a more realistic environment than being examined simultaneously by 2 examiners. Although potentially unnerving, this certainly will give you a good feel of what to expect in your exam.
The most common mistake in preparing for the OB/GYN oral exam is not having enough exposure to the oral exam environment. How many oral examinations have you had? The most important examination of your life is in a medium of examination about which you probably have little or no experience! It is vital to be comfortable with this medium of examination.