Cue dramatic drum roll. The interview is probably the thing that all Oxford interviewees fear most. Difficult to predict and thus difficult to prepare for.
I think there are quite a lot of myths surrounding the interview process, one being that private school applicants are coached to within an inch of their lives. I was one of three out of 30 non-private school interviewees at St Benet's Hall but I found that the privately and state educated applicants all had the same fears and worries and were unsure what to expect so, whilst I'm not saying that privately schooled applicants aren't necessarily highly prepped before going, the ones I met weren't and so state schooled applicants shouldn't feel at a particular disadvantage.
I only had one mock interview before I went up for my real thing. It was organised by my Head of Sixth Form and was with an Oxford graduate who now works for the civil service relatively locally. He was very helpful and invited me into a small meeting room, talked me through what his interview had been like and the kind of thing you might want to expect and then he told me to wait outside and the mock interview began with a knock on the door.
It was nice to have someone to bounce ideas and approaches off and my mock interviewer was quite helpful in asking me questions from a variety of starting points which covered an unseen source that I had had 30 mins to prepare for beforehand, asking me about my A Level periods of study, asking me about my extra curricular reading and any periods of history I was particularly interested in. Whilst this didn't mirror the real interview, I found that it did help my confidence to discuss my subject with someone who equally enjoyed it and knew a vast amount more about it than me.