Cambridge University usually sets some written tests before the interview. For example, the mathematics departments of Trinity, Homerton, Churchill and other colleges will require students to do a written test first, and then interview according to the written test done by the students.
Oxford University will have an interview for a second college. When the college receives the student's application, it will reschedule the interview of the second college according to the number of applicants from each college. This distribution is random and basically irregular. Moreover, the interviews of the first and second colleges will be arranged at the same time, which means that if you receive an invitation from Oxford University, it will be an invitation from both colleges.