When I went through OASC the first time one of the key things I remember being told before I went was that you should always asume you are being watched. One night there were a whole load of guys c***ing around in the candidates accomodation and I was in the Mess. None of those guys got through even though they were getting straight A's etc.
Also have an opinion and stick to it, doesn't matter if it's right or wrong, just have an opinion.