6 x 25 min interviews over 3 days
Problems solving and logic questions
You aren't expected to know the answers to the questions immediately, they want to know how you work it out.
Try discussing philosophical topics with someone (if you can find a willing test subject)
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.
Test taken: Mathematics Admissions Test (MAT)
Number of interviews: 6
Time between interviews: Because I was applying for Computer science and philosophy I usually had 2 interviews a day (one for CS and the other for Phil).
Length of interviews: Around 25 minutes, some shorter.
Online interview: No
Computer science was mostly general problem solving. I got asked a question which led to another more complicated one etc.. Some were based on some simple algorithms dealing with shapes, others were just maths questions.The important thing to remember is that everything the tutors ask you, you already know. You just have to find a way of applying your knowledge to the problem. Philosophy was a bit different. Depending on the college I took the interview at, the questions were either very logic-centric (on topics like induction). Others were a lot more strange. "How do you know I'm not a zombie?" would be an example (I made that one up). Any answer goes, so long as you can defend your position. Unfortunately, that isn't so true for the logic questions.
The good thing with Computer Science and Philosophy is that you aren't expected to know anything about either subject. Unless you wrote about a topic that interests you in your personal statement, you won't get any question on specific material. That's why it is definitely worth having a friend or family member read your statement and ask questions about some of the things you said. Some tutors will take time speaking to you about your personal statement, some will ignore it completely. The most important part of interviews is avoiding stress; if that means knowing your personal statement by heart, go for it. Other people will feel more comfortable by just winging it. The interviews themselves are mostly to see how responsive you are to the tutorial system. You aren't expected to know the answers to the questions immediately, they want to know how you work it out. For practice, it's worth trying to work a few problems out loud (c.f. Computer science department website for examples). Same goes with philosophy, there is no right or wrong answer, as long as you argue your point and say what you really believe; not what you think the tutor wants to hear. Try discussing philosophical topics with someone (if you can find a willing test subject).
I reworked all past papers I could find. One thing that was useful was reading the syllabus and making sure I knew all the material.
Four words: Have fun & No stress. It's mostly an opportunity to speak with super interesting people about a subject that fascinates them. If you go into the interview with that mindset, chances are you'll do better. Remember that at the end of most interviews, tutors ask if you if have any questions. You don't have to ask anything (I didn't), but if there's a specific question about the course or the field, in general, go for it! (Don't seem over-confident of course). If you got this interview, chances are you'll get into plenty of other schools, don't focus exclusively on Oxbridge. Enjoy interviews, and see where it takes you.