Well, I've completed both my interviews with Cambridge and Oxford, so I figured I'd come back and let others know how they went!
Neither of them were like the "crazy" interviews that people talk about for undergrad. None of the questions were "weird" at all. For both, I was interviewed by two people in the department; they asked me the basics of why I applied to the specific program, what experience I had, what I was interested in researching, to walk them through one of the essays I submitted as a writing sample, etc.
The interviews were still slightly intimidating because you, unsurprisingly, get the impression that these people are really at the top of their field, don't care for nonsense, and expect a lot out of you. That being said, I flubbed on one of the questions (during the Cambridge interview, one of the interviewers asked me to talk about which scholars in the field I had read, and I had to admit that I wasn't familiar with any in particular), but I still got accepted to the program, so...try not to worry too much, be yourself, admit if there are gaps in your knowledge, don't try to fake knowing more than you do, but still impress them with what you do know, and show positivity and passion for what you intend to research when you get there.
Good luck to everyone!