Two 25 minute interviews. Both only had maths questions.
Only pure maths (no mechanics / stats). Graph sketching.
Mock interviews with teachers. Practised graph sketching.
Good luck!
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.
I had two interviews over the course of one morning, and each lasted 25 minutes. I did not have to take any preliminary tests, or look at any material in advance. My first interview was relatively early, so I was offered a room to stay in the college the night before. In the morning I had breakfast in my college's Buttery (canteen), before heading to the first interview. After the first interview I spent about an hour in the college library, where tea and coffee was on offer, before heading to the second. Both interviews were purely mathematical (no personal statement questions).
One interview started with a logic question as a warm up, before getting into some harder material. The hardest part about the subsequent questions was not getting flustered when you had no idea how to get started. I found talking about my initial thoughts helped, even if I couldn't see how it would actually help with the problem. This also allowed the interviewer to prompt me when I was genuinely stuck, or let me know if I was on the right path. My questions were all pure maths (no mechanics / statistics). A large part of the second interview focussed on sketching and analysing a graph, as well as using some FP2/3 concepts.
Asked several different school teachers to give me
Good luck to anyone reading this!