PAT, 2x interviews
Maths and physics questions
Talking with friends and teachers, Iwanttostudyengineering.com and Physics and Maths Tutor
Past papers, quizzing with other people applying for my subject, syllabus
Have a good understanding of the fundamentals
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: Physics Aptitude Test (PAT)
Number of interviews: 2
Time between interviews: 6 hours
Length of interviews: 35 minutes
Online interview: Yes
No personal questions, straight into the action! Had one maths related question and one physics related question each time, and they were split into multiple parts.
Talking with friends and teachers, not even subject-related teachers, just teachers who were friendly and knew what the application process was like. They found example questions online and I got used to the interview experience in that way. Iwanttostudyengineering.com and Physics and Maths Tutor are by far the most extensive resources for engineering students, but other free resources like Professor Povey's Perplexing Problems (online) were really useful. Then just revising the very basic fundamentals of my subject - for STEM subjects, the interviewers will guide you to introduce new topics so understanding the fundamentals is more important than complex specific things.
Past papers (all on Physics and Maths Tutor); quizzing with other people applying for my subject (even if for different unis); and preparing the spec from the university's site.
The fundamentals are the most important in my opinion; for maths that's calculus and graphs. For physics that's statics/mechanics and electricity. If you are able to have a qualitative conversation (not about just maths or physics) steer it to your interests and show your passionate about the subject!