3x 20-25 min interviews, over 2 days
Interview 1 (Exeter): science-focused; Interview 2 (Exeter): problem solving, thinking skills; Interview 3 (Univ): article given day before, experiment
Past papers, books from library, A-level revision
Try to enjoy yourself in interviews and in between them!
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: BMAT
Number of interviews: 3
Skype interview: No
Interview spread: 2 on Monday about 2 hours apart, 1 on Tuesday
Length of interviews: 20-25 minutes each
2 of my interviews were at Exeter College, my third was at University College.
At Exeter, they were both quite relaxed, with one focused more on science (predominantly stuff I'd learnt at A level anyway) and the other on problem solving and thinking skills.
For my Univ interview, they gave me an article to read the day before (from a journal, I think about the impact of a high salt diet on blood pressure) and the interview was focused on that and on a second experiment that was fully explained before any questions were asked. It seemed a bit more stressful than at Exeter.
I used the past papers available on the BMAT website, and also had a couple of books from my college library, but I didn't bother with any of the paid courses/tutoring.
Make sure you know your A level content (I think this is more relevant to science subjects) and that you read your personal statement before going to Oxford.
Try to enjoy yourself, not only in interviews but in down time as well - a few of my friends now spent a lot of time in their rooms preparing during interviews, but unless you're given work to do beforehand I wouldn't really bother, it will probably only stress you out more.