2x interviews (2 interviewers each)
No reference to personal statement; questions started easy and became more difficult; stepped through calculation
Revising current stuff I was studying
Interview was more focussed on micro subjects than macro subjects
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 arrived the night before my interviews, had a meal with other applicants and stayed in a college room overnight (frantically trying to finish 'On the Origin of Species' because I'd said I was reading it in my personal statement... but Darwin is DENSE). I had two interviews
Both interviews were entirely about subject material they had chosen - no reference to anything in my personal statement (either motivation or topic-wise). The chemistry interview started really easily. I was first asked the boling temperature of water - I thought it was a trick question! But the interview rapidly progressed, each question getting harder through GCSE/AS/stuff I'd never heard of before but had built logically from the earlier questions.
The biology interview I was presented with a sheet of x-ray paper with white dots and asked what it was. I had no idea. I was sort of guided to the answer which appears in A2 biology syllabus. Then did a dilution calculation (the interviewer wanted the process rather than actually doing the calculation in my head), then finished off with some stuff about transforming cells which remains mystifying to me today.
Just revising current stuff I was studying. Personal-statement related stuff didn't come up.
This is very specific to Bio Natural Sciences - but my interviews focused on cells and chemistry.