They asked some of the expected questions ("Why Oxford?", "What do you intend to do should you come here?", "Are you familiar with the course structure and specifics?") and a few technical and semi-technical questions, but all very simple. Since I manifested an interest in Category Theory, they asked me to define "category" and asked questions such as "why is Category Theory important for Algebraic Topology?" and "In which situation have you most used CT/ What kind of experience have you had with categories?". I used that one as an excuse to talk about my latest project and I guess that was the part they most enjoyed. Since the project was about profinite groups, they asked me to define "profinite group", but that's how far as it went in terms of technicality.
It was quite informal and lasted for about half an hour. The interviewers also asked a few questions on background and experiences (for example, about my exchange period in the U.K as an undergrad) and asked me about my last-semester grades. In the end, they allowed me to ask questions as well and I asked a semi-technical question regarding one of their papers, which they seemed to like.
It was way simpler/friendlier than I had antecipated, don't panic in case you get invited, but do your best to prepare for it (become an expert on Oxford, your application materials, the interviewers, the department, your past projects, etc). By the way, I don't think you'd be able to prepare for the technical questions upfront, so just trust your background and you should be fine (they mostly ask for definitions, but I've heard of people being asked for very simple theorem statements or outlines of proofs).