It's also worth mentioning that the applications process is holistic in that they will consider every metric available to them, and any information given on the circumstances surrounding those metrics.
Say for example you are applying for English Literature, you're predicted 4As at A-Level, and got A/A*s at GCSE in English Lang./Lit., History, Geography, and other such 'essay-based' subjects, and got Cs/Bs in Sciences/Maths, your GCSEs might not hold you back so much. This shows the tutors that you have a strong aptitude for these subjects, and you compound that with having chosen them at A-Level and excelled at them. A note in your reference saying that your abilities in this side of academia are excellent would compound this, as would an excellent personal statement and admissions test. Do well in all of these and you stand yourself in good stead to be invited to interview: perform well here, and you stand a good chance of being made an offer.
Similarly, if there were extenuating circumstances with your GCSEs which the university is made aware of, poorer grades can be overlooked provided the rest of the application is good.
And if you didn't really care about academics during GCSEs and only really found your love of learning at A-Level, there are ways of saying so in your personal statement that demonstrates this to tutors. Maybe you read a certain book, watched a certain documentary, or visited a certain place over the summer, sparking your interest and causing you to really 'double down' in your A-Level studies.
The point here is that Oxford (and Cambridge, and anywhere else's) admissions tutors are very clever people themselves, and they want the best and brightest thinkers. The grade requirements allow them to cut off the majority of the population to make their admissions process feasible, but if you can demonstrate that you are one of the best and brightest despite what your GCSEs show, you are still in with a chance of admission.
Apply to your strengths if you can - as mentioned above, Cambridge is less concerned about GCSEs than Oxford - but if your heart is really set on a certain course/place, there's no harm in applying. It's just one out of your five choices, and who knows - you might get in!