A few things to remember:
1. Revise your AS an A2 work: as a bare minimum you should feel comfortable talking about the work you have already covered, should know the key concepts from the syllabus and should be ready to explore any works you claim to have studied for your A levels.
2. Re-read your personal statement! You may have written it a couple of months ago, but if you said you read Plato's Republic or Baudelaire's Les Fleurs du Mal you'd better be able to prove it. Anything you claim familiarity with in a personal statement should be a nice, easy way for the interviewers to get you started. Make sure it is.
3. Make sure you are reading something NOW and are prepared to talk about that too. A novel, a science book, history, the Economist, the New Scientist, just show you are engaging with a world beyond Facebook updates of your best friend pouting in a nightclub.
4. Make eye contact. Shake hands.
6. Think before you answer.
7. If you don't understand a question, don't be afraid to ask for clarification.
8. When you've finished answering, don't feel you have to keep talking until they interrupt. Say your piece then stop.
9. They are testing your ability to argue logically and coherently: if you find you've changed your mind as a result of the direction of questioning, that's OK - say so. Don't just cave in the moment they challenge you, but on the other hand, if you do modify your position in the face of overwhelming argument, that's OK, it shows you're intellectually flexible and teachable.
10 GOOD LUCK
See the link below for sample questions for a range of subjects from Oxford.