Its good that you're ambitious, but people with less than what you are wanting have got into oxford/cambridge.
Concentrate on your GCSEs, getting a lot of A*s, and if you really want to just one other AS level in your spare time.
And then work hard in your AS units, getting 90%s.
You don't need 8 Alevels to get into oxford, and yes, people with 3 As have got in, so breathe a little!
I also suggest do more extra curricular activities, cause the top unis like that too, you want to be a little well rounded.
But don't start worrying now about univerisity, no need to give yourself unnecessary stress, just concentrate on your GCSEs.
Any tips i can give you for as levels is make sure you know everything on the spec, and lots of exam qs practise , same with GCSEs.