First of all, have you looked at any courses on the Oxford or Cambridge websites?
For Cambridge, you will not usually find any GCSE entry requirements apart from a pass in English and Maths. Cambridge will look at your GCSE grades but they have no set criteria.
Oxford do, and they vary. But they aren't all 8s or 9s (except Medicine)
As the user further up said, make sure the circumstances about your arrival to the UK are included in your UCAS reference
And remember that Oxford or Cambridge is just 1 out of 5 universities you can apply to in UCAS.