Interview 1: Image description, Latin unseen and conversation around a prepared topic; Interview 2: Comparison of translations and image for discussion
Believe in yourself
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.
Number of interviews: 2
In one they showed me an image to describe, asked me to translate a piece of Latin unseen, and had me talk about a topic I had chosen before. In the other one, they asked me which of the two translations they had given me was the best one for a passage and showed me an image for discussion.
I revised my grammar, read books, and practised speaking about the topics I liked.
Believe in yourself