2x interviews (30 mins each); 1x test (30 mins)
First interview: longer questions; Second interview: taking test content further
Mock interviews; IWTSE; Maths/Physics Olympiad questions; Professor Povey's Perplexing Questions; going over school work
Fluency in maths is key
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.
30 minutes general interview (long challenging questions); test (30 minutes - fairly standard A-Level content - time pressured); 30 minutes second interview (to discuss test questions and go beyond the questions to discuss more challenging concepts)
Essentially a conversation where you discuss your thoughts and ideas as you attempt to answer the questions - the interviewer may steer you in the correct direction if required - never forget to say what you are thinking!
Interview questions touched on areas including density, some circuit analysis, integration, graph sketching, fairly easy complex number stuff, mechanics questions and some probability.
Fluency in maths is very important.