Based on my experience at OBA, I would recommend the following if you anticipate going to OBA.
First purchase the study pack and study for the exams before going to OBA.
Then when onsite at OBA, you should take the exams there. Thats included in the fees anyway. At the same time, interacting with others onsite might help you pass the exam easier. Make sure that you pass the exams within 2 weeks of your arrival. Thats actually possible if you have really studied for the exams intensively. Dont forget the confuser. It is a valuable resource.
Good luck.
WP