The importance of GCSEs in Oxford applications

A 6 in GCSE Art would not matter if your application is otherwise strong and aren't applying to the Ruskin School of Fine Art.
Oxford tutors aren't obsessed with GCSE grades, but a good overall performance at GCSE is seen as a an indicator that a student has academic focus and promise. Tutors expect schools to over estimate A Level grades. The subject-specific aptitude test is a big factor, and the interviews are the make or break point, if you get that far.
Work hard, don't burn out, good luck!

You are wrong that Oxford 'expects schools to over-estimate' A levels - they know which schools are accurate and which are not. I am held to account for my predictions as is every other teacher in my school. That means our predictions are almost 100% spot-on and so few miss their firm offer.

The words that you quote are the words of a senior fellow of an Oxford college.

I’m applying to Oxford this year but I’m slightly worried about my GCSE grades.
Most of my grades are good (8-9) but I got a 6 in art, and whilst this isn’t the subject I’m pursuing, I have seen various sources on google and on social media that have worried me.
Oxford said that most of their students get all 8s and 9s (perhaps an odd 7 at the very least) - so will a rogue 6 work against me or am I being extremely paranoid?

I got a 6 in GCSE art and I got an offer this year. You don't need straight 9s to get in, and as your other gcse grades are good they're unlikely to impact your application. A-Levels, ps, entrance exams are more important