I had two 25 minute interviews and the CSAT test (I think it will be 120 minutes long for future applicants). Some applicants did the test then interviews, with a break in between, some did the opposite.

Both interviews were entirely technical; no small talk or questions about my personal statement at all. The first interview had two multistage questions about logic puzzles. The first one went well after an incredibly stupid initial mistake, the second went less well but I arrived at the answer eventually with some prompting. The second interview consisted of a few short maths questions based on A-level content which I found easy, and a longer question on a theoretical computer science topic which I luckily knew about.

The CSAT website has a lot of resources linked which will also be useful for preparation for the interview. Personally I found UKMT Senior Maths Challenge and MAT papers most helpful. The Oxf*rd Computer Science website has some example questions that are similar to the ones I had. My school organised a practice interview for me, but I didn't find it that helpful although it could be useful if you think you'd be especially nervous in an interview situation.

Especially at the start of the interview, don't rush into things and make stupid mistakes due to nerves. While it is true that successful applicants often feel like their interviews went badly because the interviewers try to push you, this can be overstated; they will generally try to put you at ease as well.