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.
Number of interviews: 2
Skype interview: No
Interview spread: all in one day
Length of interview: 25 minutes
The interviewers were nice but I was very nervous so I answered questions quickly, giving the wrong answer, which is definitely not the way to go about it. They didn't ask me anything about my personal statement, why I wanted to do chemistry, or anything about me really. They just asked chemistry related questions.
It was very stressful but you have to try and take time to think and say something useful/related to the answer, rather than just rush to give an answer that you haven't thought about. We did some physical chemistry, some organic and some inorganic, and then some maths. Some of it was stuff we had covered in A Level but a lot of it wasn't. They walk you through an example and you give ideas as to the answer for a similar question.
I did all the past papers and tried to find trends in the answers. It was dull but practice definitely helps and I did quite well in the test. It's
Nerves can easily get the better of you, so just try to relax and to think thoroughly about the questions rather than to rush to give an answer which you haven't really thought about