Most subjects do have a specific additional test nowadays, but it will depend on which subject you’re applying for.
They will want at least 3 AP exams with scores of 5, as closely related as possible to the subject you’re applying for; excellent recommendation from your teachers; and a convincing personal statement regarding why you’re a good fit for the course. GPA isn’t relevant per se, because they know it can be subjective. You apply via the UCAS application which all UK universities choose, and you can use the same application for four others as well (excluding Cambridge - you can only apply to one of these two). You may also need to register for additional Oxford tests as above. Deadline for Oxford apps is 15 October.
If your application passes the first cut you will be invited for an interview with academic faculty members, which is then the deciding factor.
What subject are you interested in? The Oxford website is reasonably clear about minimum requirements, timetables, etc.