basically just make sure you understand exactly why you're doing an mba at this point in your career, and why oxford in particular. know your resume and essays and be prepared to speak to any weaknesses in your profile.
also they threw a completely oddball question at me, and i've heard from some others they got different weird questions. so maybe you'll get one too. my advice in that case is to be calm, collected, and logical in your response - no matter how weird the question is. you can take a moment to collect your thoughts - the interviewer knows it's an odd question and they're not going to expect an answer right away.
otherwise, good luck to all those who got invites.