DD is in her 3rd year reading French at Oxford. It was her idea to apply and I was thrilled to support her. She is studying linguistics (for the first time) as an option, not as joint honours. The teaching across the degree programme is rigorous and challenging. There is a huge amount of reading to be done, grammar, speaking and translation classes, and fortnightly essays, written, uncommonly for an MFL undergraduate degree, in English. Hard work, as it should be, but DD is having an absolute ball and has found an environment that suits her in every way imaginable.
At A level, DD was awarded A, A, A, with the A being in French. A for a non-mother tongue speaker is rare, so hats off to your DD for her predicted A. GCSE results are not a major factor when applying. DDs were all 9s, 8s and 7s.
Modern Foreign Languages is one of the least competitive subjects for which to gain entry to Oxford, with about half of candidates interviewed being offered a place: https://www.ox.ac.uk/admissions/undergraduate/courses/course-listing/modern-languages. This page also lists colleges offering French and Linguistics joint honours.
Does your DD's school have experience in supporting Oxbridge entry? If so, they should be able to advise on choice of college (in my DD's case, this was to do with her now college's established links to our part of England) and whether your DD's ADHD needs to be mentioned by the school in her application.
The application process involves sitting the MLAT (Modern Language Aptitude Test) - past papers are available online so candidates can practice - and a remote interview. As a PP said, literature is a large part of the course, with philosophy, plays and film featuring too.
What else should your DD do? There's an open day on 11 May - details here: https://www.mod-langs.ox.ac.uk/schools-liaison-and-outreach/meet-us-person
At interview, candidates need to persuade the interviewers of their unwavering dedication to and passion (I'm squirming as I heartily dislike this slovenly use of that word) for the subject. Showing engagement with the language beyond the A level syllabus is really important - reading, listening, watching, speaking - as a way of communicating that dedication to the interviewers. So your DD should start now if she's not doing so already, and keep notes of her views and reflections in preparation for (hopefully) an interview.
Good luck to your DD! It's really good to hear of a young woman ambitious for herself 😊