2x interviews, 1 day apart
Open questions, general discussion
Past papers, reading
Just think about your subjects and why you want to study them at Oxford, and keep reading.
Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Number of interviews: 2
Skype interview: No
Time between each interview: 1 day
The interview was a fantastic experience, not so much involving any targeted questions expecting textbook answers, but rather generic questions that allowed me to guide the interview in whichever way I felt most comfortable. It was a discussion of ideas, quite general at first, but which I responded to with specific ideas that I felt keen on developing. It was, in short, quite relaxed, a fun, intellectual discussion that did not feel at all like an entrance exam.
I did past papers and some outside reading.
I simply thought about my subjects, why I really wanted to study them at Oxford, and continued with my wider reading. That is all you need.