Oxford and Cambridge may require you to take an admissions test as part of your application for Philosophy. Always check the relevant website for up-to-date information on entrance requirements.
See Oxford Admissions Tests for more information on which courses require which admissions test.
You can find information about the Cambridge admissions process on their department pages which contain course information, recommended reading and a link to a specimen written test.
If called for interview by Oxford, Cambridge or another university, there are things you can do to prepare.
The most likely topics of conversation will be those areas you have mentioned in your Personal Statement. If your EPQ involved Philosophy, or you made reference to A Level topics, these are also likely to be discussed. You should be prepared to speak for a couple of minutes on any of these.
Other topics might include philosophical books you have read (if they ask what you have been reading recently, are you able to answer with confidence?), or perhaps your favourite philosopher or even your preferred theory in one of the areas you study (eg are you a utilitarian? a dualist? a Wittgensteinian?). In all cases be prepared to justify the answer you give.
You should also be familiar with the course for which you are applying. Not in too much detail, but enough to be able to say why you have applied for the course, and what you are looking forward to. Similarly, you should be able to say why you have applied for this particular university or college. At the end of the interview, you may be asked if you have any questions of your own.
Practice interviews can be helpful, particularly if they are challenging. Learning how to respond to difficult or puzzling questions is very important: you might ask for clarification, for instance, or seek to relate the question to something more familiar.