Sixth Term Examination Paper (STEP), 2x interviews
Questions of increasing difficulty
Doing mock interviews with maths questions
Past papers
Try to present your best self and your passion
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: Sixth Term Examination Paper (STEP)
Number of interviews: 2
Time between interviews: 5 mins
Length of interviews: 25 mins
Online interview: Yes
It was easier than I expected, and when the only challenging topic appeared the time was up.
Finding some maths questions, and doing mock interviews with myself.
Past exam papers.
Try to present the best of you and be passionate about your subject (even if you are not).