I very nearly did get into Oxford with very poor GCSEs. I had 1A* at GCSE, INCLUDING A C IN MATHS!
I applied to Keble College for Philosophy and Theology and was chosen to come to interview for 2 days (8 people were chosen out of a total of more than 28).
So, despite my abysmal GCSE results, how did I make it this far in my Oxford application? Well, firstly I smashed my AS Level's. What Oxford admissions tutors like is someone who has what they call a high 'potential' for progress. So that's someone who's basically done bad at GCSE but extremely well at AS and A Level (only problem is, if your school does the new A Level and enters you for the AS level at the end of year 13, it's less likely if you apply to Oxford you'll be considered for interview because the most recent grades they have to go on are your GCSEs. In that case, when you do smash your A Levels, I'd consider taking a gap year then applying.
Secondly, I smashed the Philosophy admissions test... And believe me, it was extremely hard. And that's the thing - you can have 100A*s but if you don't score impressive marks on your admissions test then you're screwed mate.
Thirdly, my personal statement was very impressive - a lot of time was invested in it.
So overall this got me to interview, but that was the stumbling block for me - interviews were just too tough for me - I left them knowing I haven't done well. And with only 3 spaces, you can't afford to slip up. If you do want any chance of getting in if you get an interview, here's the advice I would give you - BE YOURSELF!