Good Evening Dan,
If you look at the thread regarding OASC at the top of this forum, there will be hundreds of tips and advice in there for you.
I myself am 18, and have recently applied for WSOp, however I finished my A levels early and was only awarded 2 x AS grades, so best advice is to stay on and complete your full A levels (unlike me
).
As I have been told by many on this forum (Who's advice is priceless by the way
) get good leadership experience and try and get yourself in a sports team.
Hope this helps, but please be aware I am only a wannabe also there are many current and ex. Military aircrew on this forum who I am more than sure will be happy to help.
All the best
NDW