MAT; 4x interviews.
Varied set of maths-based questions; struggled but was given help by tutors.
Practice interviews; watching mock interviews online.
Preparation is important; relax and enjoy the interviews and make the most of it.
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: Mathematics Admissions Test (MAT)
Number of interviews: 4
Time between interviews: Min 4 hours (roughly)
Length of interviews: 20-25 mins
Online interview: No
They didn't ever really ask about my personal statement or general questions (occasionally 'why do you want to study maths?' or 'why you enjoy maths' but very short at the start). One asked me for my 'favorite topic' and asked a question on that. Common questions are graph sketching, calculus (integrals) and mechanics. Some are given a problem sheet first to tackle (though I wasn't) and discuss in the interview. I was nervous at the start but if you find the questions engaging and interesting you become more relaxed and focused, like a real tutorial at Oxford. One interview I struggled in particularly because I didn't understand completely what the interviewer was asking until near the end, so make sure to ask any questions if something doesn't make sense and if you find you're getting stuck at a certain step they can guide you (again like a real-life tutorial).
Practice MAT papers during half term, STEP 1 questions, practice interview questions (from Student Room, YouTube, StackExchange etc.)
MAT: Do as many past papers as possible (under timed conditions) - but also practice STEP 1 (and even STEP 2) questions - so be very familiar and strong on first year A-Level Maths and Further Maths. Interview: Again be familiar and strong on first year A-Level maths and further maths, and also any content from the start of year 13. Can begin looking at more STEP 2 questions and several past and practice interview questions online but practice with a teacher or friend talking through your working and solutions out loud. During the interview; the whole experience is smoother, less stressful if you enjoy it! Make the most of being able to discuss quite interesting maths with some of the best tutors in the world. And feel free to ask any questions about what you covered during the interview at the end if it has sparked your interest (and if there is time).