The DM jumping on the bandwagon of course re ADHD as part the current narrative about over diagnosis of conditions that are not immediately visible, be that physical or neurological. I can't comment on Oxford's internal processes for assessment of ADHD but it is interesting that overall they seem to have a significantly higher intake of people declaring disabilities (19%) vs Cambridge (13%). In terms of offers at Cambridge though, I would say that people applying with autism and SpLD eg adhd & dyslexia make up a below average % - 1.6% of offer holders declare autism and 2.3% a SpLD.
It is however quite possible that a significant number of students arrive at Oxford and Cambridge (and other top unis) with undiagnosed neurodiversity incl ADHD, particularly academically able girls who have quietly struggled with focus and taken forever to get their work done but somehow always deliver, with resulting impact on mental health. I have a DD at Oxbridge who was diagnosed with severe ADHD in her teens (on meds and 25% extra time) but who could so very easily have slipped through the net because she was still achieving at school and was seen to be a model pupil. Undiagnosed, these students are likely to fall apart at university once they no longer have structure and parental oversight. It's possible that many of these students may be being picked up at Oxbridge where it is harder to hide and where difficulties may be picked up earlier because of the tutorial system?
I would second @StrawberrySwitchcake 's point that it is pretty hard to survive at Oxbridge with ADHD unless really motivated, but also that as @MrsBennetsPoorNerves says, the weekly deadlines and tutorials provide a structure and pressure that is needed. The first 2 terms have been pretty stressful and exhausting for my DD but she is still happy she's there. Long deadlines with everything due at once / end of the term would be impossible for her to manage as she just wouldn't be able to finish anything until the last minute. Managing basic 'adulting' like buying food, hobbies/societies and dealing with emails etc, on top of studying, is another layer of 'stuff' that can cause neurodiversity to become more overwhelming at uni.