@Rainbowglassbowl very good luck to your DC, who'd clearly thrive, if Oxbridge are lucky enough to benefit from their presence!
My two DCs got considerably lower GCSE results at a school which always had very high grades but then went on to do really well in A Levels. I think at GCSE, they each got things like 4 to 5 A*s, 4 As and perhaps a couple of Bs, although I now can't fully remember.
They applied twice - firstly, not getting offers after getting interviews on the first try. Once the A Level results were in (this was all pre-Covid, when they did the proper public exams in subjects they loved and got A*s) and they did round two of Oxbridge applications and interviews, both got places. A Level grades superceded GCSE grades.
Many of their peers who'd got much better GCSE results, didn't make it and some did and I do think that once you get to interview and have done well enough at the entrance tests, it was about who the college/ department thought would thrive most and really enjoy the teaching methods.
I recently met up with some old friends from my own Oxford Uni days (42 years since we were at interviews) and we were chatting about our experiences. All of us hadn't got stellar academic results but seemed to do something 'right' at interview:
I got every single interview question incorrect but then got into a very interesting discussion about one of my extracurricular pursuits, (art) in which my tutor-to-be's husband was a professional 'big-wig'. So that went down well. I also came from a school that barely sent anyone to university at all and never to Oxbridge.
One of my friends got angry and assertive with her tutor-to-be (same person as mine) at her interview and this was considered very positively, as our tutor liked a good 'fight' academically. Looking back, I think it was all about being able to have a discussion, express an opinion and back this up, more than a perfect set of exam grades.
Whilst things have changed a lot over the years, I think that there's really nothing to worry about if there are less than a perfect set of grades, as there's more to the selection process than just that one aspect.