Firstly I can say that general questions ‘Why do you want to study at Oxford?’ or ‘Why do you want to study Chemistry?’ are often not asked. Ok there might be some exceptions but I can say that on most cases, there will not be such questions. We go straight into the academic questions.
However, the first question often tends to be easy, doable ones like A-Level standard even AS standard, IB standard. The examples I’ll be giving will have to be biochemistry based because I studied biochemistry myself. But if I give you an example, my first interview’s first question was to draw an amino acid. That’s quite a simple one, biology AS level. There was a whiteboard next to where I was sitting and I was just asked to stand up and draw it out. We started with this easy question and then started building up some difficulty to what I feel was university first year level. Just to give you an idea, after ‘draw amino acid’ question, the next question was to draw a peptide bond by drawing another amino acid and joining them up. Then the trick question was the one after. It asked me what is the most electronegative atom in this peptide? Thinking now, it is not as tricky as it was in the interview. But in that time because I was under pressure, I was very nervous, this question really tricked me. Obviously it’s oxygen but I was trying to find whether it is nitrogen or oxygen. But yeah I chose oxygen correctly. And towards the end, towards the more difficult end questions, (I will just give you an example), the question I got was “If a codon has 4 bases instead of 3, how would this change the structure of tRNA?” Definitely something you don’t study in A-Levels and something that most biochemistry applicants will not have immediate answers to.
But hey, now days you can find a good wealth of past interview questions by googling. Do make sure you are solid with all the biology and chemistry topics covered at school before the interview. I discourage trying to prepare for the really hard end questions and neglecting the simple AS/A2 level stuff.
There are several ways you can prepare for interviews. But I will leave what I think is the most effective for subjects like Biochemistry. Not everyone is good at orally explaining concepts (Including myself). What I used to do is I drew up a long list of chemistry topics like 'DNA replication', 'nucleophiles', 'cloning', randomly pick out one topic and practice explaining out loud for 1-2 minutes.
The mission was not to say 'uhm' or stop in the middle of the explanation to think of what to say next. This was to deliver more logical and coherent oral answers in the interviews. I've done this for a month, not necessarily everyday but good 8 topics a week. Hope this helped!