OSB's are tough, especially if you have them the day after FSP finishes. The main advice I can give i'll give in points.
1 - When asked maths questions by the interviewers, do the working out loud. So, if they said "what is 10 multiplied by 120", you'd say something like "well, 10x12 is 120 then add another zero, so 1200"
They want the working as well as just the answers.
Also, when it comes to questions like "if you were on the edge of a cliff, would you jump off?" - don't see it as a retarded question that they use to see if you're psycho. Instead of saying 'no' like everyone seems to, give an answer which seems like you thought about it, like "well, if i had a jetpack or a parachute I'd consider it, but not without any device like that"
Oh, and address all the people on the board by RANK, so many fags walk in and say hi, then just sit down. Learn all the ranks for your service, both officer and general entry, and greet them by their ranks. It gives a great impression.
All in all - be calm, and be truthful. Good luck.