I would agree with you that the selection at Oxford is hard, I was the only one on my selection to get through, well first time anyway.
I am doing Physics, History and Spanish with an AS in maths. I would not say that the A-Levels that you do are all that important as they are only really viewed by the FTO and airlines, will look at them, but will be more interested in your flying ability and personality.
What are you doing?