How should I prepare for an Oxbridge philosophy interview?

The main thing to remember when preparing for your interview is that the interviewers are just people. They want to check that you are genuinely interested in their subject and that you wouldn't be a nightmare to teach! Think about it, they have to sit down with you for an hour each week, they want to like you. So try to relax, be friendly and don't second guess them, or yourself. Interview questions in philosophy are rarely questions of factual knowledge, typically they are just trying to see how you think. To assess this they will often give you a jumping off question such as, 'is it ever ethical to kill a man?' and then will see where you run with it. The key with these questions is that there is no right or wrong answer, they will play devil's advocate to test your ideas and to see how well you process arguments. Don't be afraid to take a minute to think, or even to admit that you have begun to disagree with a statement you made earlier, this approach will often impress the interviewers. In terms of preparation, I would advise having a look at an introduction to logic book, this is the only element where some knowledge may come in handy. I would also suggest perhaps reading or skim reading an introduction to moral ethics, as this is a popular area they focus on (mostly because discussing it does not require much pre-existing knowledge).