My school paid for me to go on an "Oxbridge Admissions" day, where they claimed they would teach you the secrets of getting the interview right. They claimed they had specialist inside knowledge of these interviews. The day boosted my confidence, but the practice interviews we had were *nothing* like the real thing. You have no idea what they're going to give you to work on, everyone I spoke to said they were so different to what they expected/practiced. An exam is going to be a variation on a set syllabus, with many questions being the same. They are not confined to a syllabus in Oxford - they can ask you anything. To answer my interview/test questions 90-100% correctly, I think I would have had to have completed a Psychology degree in the first place!
If anything, your preparation day should have told you that it isn't just about getting the "right" answers.