4x 20 min Maths interviews; 2x 20 min Philosophy interviews (1 with 10 mins preparation); spread over 3 days
Maths interviews: problem-solving; Philosophy 1: logic, discussion; Philosophy 2: discussion
For the test: look through aspects of the curriculum not covered in school; practise past papers under timed conditions
Check the information on the university websites; practise thinking through difficult questions; make sure you look after yourself, physically and mentally; your background isn't important, the only thing that matters is that you're passionate about your subject(s)
Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Number of interviews: 6
Skype interview: No
Interviews spread over 3 days.
Length of interviews: 20 minutes each
I had both Maths (4) and Philosophy (2) interviews.
In the Maths interviews I got some problems and needed to solve them together with one of the two
The Philosophy interviews were a little more stressful, probably because I feel less confident about that subject. The first one was mainly about logic. The important thing is to argue clearly and to respond reasonably to the tutors' objections (either by defending, adjusting or changing your position). In the second one, I got a problem to work on for 10 minutes before proceeding to interview, where we discussed my solution.
I can't know this for sure, but they didn't seem that interested in my personal statement. While I was asked about it in most interviews, I had the feeling that this only served to relax me by giving me an opportunity to give well rehearsed answers.