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: 1
Skype interview: Yes
Length of interview: 1 hour each
I was given 2 physics questions and 1 math question. I solved them and the
Don't panic. Solve the problems in the interview and you'll stand a good chance of getting in.