Ricky, not trying to sound condescending. First point is your OSB was not 15 min long. The interview stage of the OSB was/may have been 15 min long. You may have some questions about the length of this but don't forget to task into consideration that you have been observed for the past 12 days. Your instructors have also spent some time with you and would have provided feedback. So arguably there may not be a lot to discuss at interview which would also, time wise, be impacted upon by your service preferences (not interested in a service then no questions from that service advocate).