First, don't panic. Second congrats on getting an interview. That's an achievement in itself. Third check out the handy guide to interviews on the Oxf Uni website
Interviews | University of OxfordIn particular look at a) the sample questions and b) demonstration interview. This will give you an idea of what to expect.
The tutors will be looking for how you think, so make sure you explain your thinking, and focus on the questions being asked. Bring in any background knowledge as and when you can, but as you can see from the examples they will be asking unfamiliar questions that make you think on your feet. It's ok to think about the question for 15 - 20 seconds but you should then really launch into your answer, with something like "my first thoughts are.....". Don't be afraid to break the question down into bit sized chunks, instead of tackling it in one go.
Oh and read through your personal statement the night before the interview. They sometimes like to begin by talking about your PS. Good luck.