2x interviews (1st with 2 professors, 2nd with 1 professor); each 20 mins.
Putting A-level physics, chemistry and maths into a new context.
Past papers; mock interviews to practise verbalising thought processes.
Generally the problems are simple; no magic required; think about case concepts; give yourself time to think before answering questions.
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.
Two interviews of 20 minutes, one with two professors, the other with one professor. One was at 11am and the other at 5pm and I came for the day but didn't bring a book or anything to do so I sat in the colleges library common room and chatted a bit and was bored because 6 hours is a long time. I didn't have to do a test.
The interviews involved working through A level physics, chemistry and maths with a new context to think about. They wanted me to think about the context to the maths I was doing and why it worked that way. In chemistry
Doing past papers on maths content I'd already learnt to reinforce my knowledge meant that the mathematics part was easy and natural, which gave me time to think about the physics of the system.
Remember that generally the problems are simple, there is no magic required of you, just think about the base concepts. Also always pause after they've asked a question to think more, it helps keep you relax and even if you blank out and think about stars during it, it will make you seem more intelligent.