My two biggest tips for OSB are;
To wait and think before you talk. It seems obvious but the nerves on the day will tend to make you rush and say something incorrect, or just inappropriate.
The other tip is once you've said what you need to, shut up. The sound of silence is daunting especially when you have four potential employers staring at you. They're waiting to see if you snap under the pressure of silence and reveal too much.
Be clear, concise and take your time. The greatest pilot of all time, Neil Armstrong would never say something to which he would later regret. In the air however, he was as sharp as a tack. Don't assume that because you emit a slow, methodical demeanour the board will assume you're slow to react in the air. They're looking for officer qualities remember.