Since we're on the subject of Oxford, I recently met with Eleanor Reeves (admissions officer of Oxford). The placements are around 1 in 5 people who get in, so you have to be excelling yourself. However, they focus greatly on A-Level results and a set amount of A* in GCSE isn't required - it's all about your potential and ability to continue to strive and be motivated. I'm currently in Year 10 and am also looking to get all A* in GCSE or at least 9.
Also, extra curricular activities do not matter, Oxford focuses on academic ability alone. However, they expect you to be extremely interested in a particular subject for you to have read a book on it and done stuff in your spare time. So in a way, extra curricular does matter but only if it's very relevant to what course you take.
Hope that helps :P