TG172. The interview at OASC is a filter tool to get you through to part 2 of the selection process. That too is at Cranwell, and it forms the main part of the assessment. As I mentioned on another thread, being fit is a simple way of improving your chances of selection. The "Bleep Test" which you do on day 2 can put you in with a shout of selection if you have above average fitness or then again, take you out of it if you are unfit.
In the interview be yourself and do not try to bulls**t. The guys will see through you and allow you to start digging a hole to bury yourself in. It helps if you've done D of E and have done some adventurous training but it is not the be all.
If you are near to a UAS, give them a bell and ask if there is the possibility of a visit. You might learn a lot more than you can through this thread.