Staircase 12 is a site run by my old Oxford College containing resources for high school age students applying to Oxford or other top universities.
The book reviews, written by students from University College, are a great place to find recommendations for interesting and enjoyable reads to extend your subject knowledge. (Which is a great things to talk about in your UCAS personal statement!)
I watched a lot of Simon's Oxvlog videos before applying to Oxford myself. His channel has a lot of useful resources for applying to study Physics at Oxford, including interview questions and other admissions advice.
Take a look at his 'Oxvlog' and 'Oxbridge Admissions' playlists!