HAT; 2x interviews.
Interview 1 (History): written work discussion, pre-provided source analysis; Interview 2 (English): source analysis, written work discussion, felt more relaxed.
Re-read personal statement and submitted work; chatted with other prospective students.
Keep calm and enjoy the experience; think out loud; don't be scared of saying something silly or a wrong answer.
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: History Aptitude Test (HAT)
Number of interviews: 2
Time between interviews: a day
Length of interviews: History was 20 mins, English about an hour
Online interview: No
In both my interviews we talked about the written work I'd sent in, and analysing a source - the history one I'd had some time outside the room to read and make notes on, while the English one was given to me in the room when it was time to talk about it. I was asked questions about my submitted essays, and if I still agreed with what I said, why/why not etc. I was very stressed walking into the room both times but I was quickly distracted from that feeling when I was thinking about the questions. My english interview felt more relaxed and like a conversation than the history one, but you do get into the swing of it!
I reread my personal statement and the written work I'd sent in- this is helpful as you can prepare yourself for any questions regarding them, especially if it's been a while since you wrote them. The best thing I did though was to chat with other prospective students (or, if you're online, anyone around!) just to destress before the interview. Keeping yourself calm is key.
I watched a video put out by the university about it- this should be on their page for the HAT. I also completed several practice papers- some I talked through out loud with someone else (or to myself!), some in exam conditions. If you can, getting them marked by a teacher is useful, but not necessary so don't stress!
My advice would be to try and keep calm! I know it's hard to do, but the interviews are nothing to be scared of. If you like your subjects, this is just a chance to talk about them with other people who also like them! Talk to people, have a cup of tea, and don't try to predict what's going to be asked because you will likely just stress yourself out. In the interview itself, try to think out loud - it's not about whether you're right or wrong, and it's okay if later you realise you said something silly (I definitely did!). The important thing is letting the interviewers see how you think and what ideas you have. I also think expressing your curiosity and willingness to adapt and change your way of thinking is very good. However, if you believe something, you shouldn't be afraid to argue for it just because 'the interviewers know better'- it's good to have opinions, as long as you can back them up.