OASC
Littleme
Having myself fallen into the OASC (Biggin Hill) trap of being convinced by the DS that I had a higher aptitude for FC than ATC (my first choice) and being too tall for the diminutive Jet Provost, despite passing for GD(P) and (N), I tried to change branch once I had graduated from Cranwell and on my holding post - it seldom happens and you wouldn't want to have to go back to OASC for reselection if you were unsuccessful in training and if it had been evident that you hadn't been heart and soul in trg.
I was 18 when I went to BH and still look back and think how did I get through compared to many of the candidates that I know failed selection. The main thing is to be confident, this is mirrored throughout all of the above posts, and while being assessed as a leader, be a part of the team.
However, I wouldn't encourage anyone to make any form of career decision on selecting any branch based upon brochures and AFCO-speak - no matter how good and honest the staff, you are making a life choice. Ask the CIO for a visit to a working unit (they used to be called realistic job preview visits) - in your case I suggest to a main flying base where you can meet officers from all branches in the workplace. This normally involves staying overnight in the mess and visiting any areas of interest to you as well as getting more info on training. If nothing else, you will catch up on many OASC stories on selection boards and give you a chance to discuss issues with the RAF outside of the AFCO/OASC.
Good luck