Philosophy Faculty admissions test; 2x interviews
Interview 1: philosophical problems; Interview 2: philosophical problems
Read a few introductory books on philosophy
Practised ways of approaching essay questions
Focus on thinking about philosophical questions rather than reading philosophers' thoughts
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: Philosophy Faculty admissions test
Number of interviews: 2
Time between interviews: 1 hour
Length of interviews: 25 minutes
Online interview: No
Both my interviews started with quite random but seemingly simple philosophical questions before developing based on the answer I gave. It was quite similar to a Cambridge
I read a couple of introductory books on philosophy and, before the interviews, looked over notes I'd made on anything I mentioned in my personal statement (although none of the reading I did was brought up in the interview, nor anything else from my statement). I read all the official university information about what interviews are/what they're for (and not too much Student Room etc.). I think I made my family ask me philosophical questions of my choosing so I could deconstruct them and think about them aloud!
The test was made up of a short, multiple choice logical thinking test (20 mins) and a short essay (40 mins). I found an example test on the faculty website and had a think about how I'd answer the questions. I didn't do loads of preparation; the point of the essay wasn't to test my knowledge but to see how I approached the questions (much like the interviews). I think I was even told that the test didn't require any prior philosophy knowledge.
At this stage, taking time to think about philosophical questions and how to approach them is probably more important than reading lots about others' thoughts. Think aloud in interviews and take your time. Don't trust the confidence of other people you meet at interviews (or, for that matter, on things like Student Room forums -- these always convinced me I wouldn't get in).