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.
Test taken: None
Number of interviews: 3
Time between interviews: About a day
Length of interviews: 20-30 mins
Online interview: No
My first interview was split into two sections, each led by a different tutor. They took the style of a short tutorial, with the tutor introducing a concept to me and then asking various questions, such as asking me to sketch what I thought might be an appropriate graph for the situation or to suggest a technique that could be used. When I didn't know the answer (or was going down the wrong path) they would guide me towards the answer with helpful hints. These were definitely not intended to be a way to say "you're wrong" so much as "that's a good idea, but have you thought about this?" so it doesn't mean the interview is going badly! My second interview was slightly harder as I got stuck more often due to a few misconceptions. It was also in this interview I got asked a few questions about my previous academic record and my personal statement. The interviews are all quite short, so there were many times I felt I should have gone into more detail. My last interview was based on an article I was asked to make notes and answer some questions on before the interview. I was able to discuss answers in the interview that I felt I hadn't got quite right, which was a helpful way to explain what my thought process has been. I was also asked about the book I'd mentioned in my personal statement, leading to quite an interesting discussion.
The most helpful thing I found was practising explaining some of the science topics from my A-level to other teachers or my family- it helped me to get used to explaining my entire thought process, even details I originally thought too small to mention that are actually quite important! Right before the interviews I also made sure to review the notes I'd made on the books I mentioned in my personal statement so I could remember enough to talk about if asked in my interview.
I would definitely say for interviews- keep talking! No matter how uncertain you might be, what the tutors are looking for is how you think, not how quickly you get to the right answer. Don't be afraid to ask for clarification or to get stuck on a question - most people I know here had those moments in their interviews!