Any time in the year before you would hope to start the degree. If you haven’t taken them by the time you apply don’t worry - just say in your app you’re taking them, get your academic reference to give you predictions of scores, and arrange to get the scores sent to Oxford and any other UK universities you’re applying to. If you get an offer, it would likely be dependent on your getting 5s in those AP exams. They’re accustomed to UK students not getting their exam results, and being able to confirm acceptance, until late August.