How do I prepare for an Oxford Chemistry interview?

Start by checking out this webpage with a sample chemistry interview question and useful videos
https://www.ox.ac.uk/admissions/undergraduate/applying-to-oxford/guide/interviews

Oxford interviews are entirely about academic questions. They may begin with a question from your personal statement but that's just a warm up question to get you comfortable. Read through your personal statement the night before to remind yourself about the things you wrote. It's meant to be a discussion so it's okay to ask questions to clarify things. I don't have a personal experience of chemistry interviews, although I did see a mock interview that had a question about the 3D structure of organic molecules starting with methane.
The interviewers are looking for a good background knowledge in chemistry to A level standard, curiosity, some evidence that you've gone beyond the A'level course to find out more about chemistry, critical thinking and problem solving, being able to adapt your knowledge of chemistry to unfamiliar settings.