What other people said ^
Also, in answer to your question - did you have extenuating circumstances, so these grades do not reflect your true ability?
If yes: Move onto AS levels / A levels - don't retake your GCSEs. When you apply, make sure your school includes a detailed letter as to why your GCSE performance was below expected.
If no: Move onto AS levels / A levels - don't retake your GCSEs. Oxford will not accept your retaken GCSE grades at the same value as someone who got them the first time round. You will not get in.
There are fantastic universities out there which aren't Oxford - and if you focus during sixth form, you may do well enough that Oxford *might* make an exception for your GCSEs.
Also, don't pick a course because it's less competitive... You should go to university because there's something you want to study, not because you want to go to Oxford...