3x interviews (25 mins each) with 20 mins of preparation material, 1x test (1 hr).
One interview for each language (speaking & grammar qns), plus a general interview discussing motivations and interests.
Understand why you want to study your specific languages, read relevant books and other materials and develop thoughts about them to discuss
Be 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.
3 interviews, each with 1 interviewer, lasting 25 mins each and each having about 20 mins of preparation material to review just beforehand. This was one interview for each language, plus a general one discussing my motivations and interests. I also had a test which I think was 1 hr in length.
Unseen Spanish poetry (an author I'd never heard of), unseen English poetry (very short verses), grammar exercises. Quite straightforward, made to feel comfortable with everything, questions relating to poems mainly about its meaning so quite accessible. I think the interviewer also then released some more information about the poems, in the light of which I reviewed/refined my initial interpretation. The grammar questions were normal gap-filling exercises: tenses, irregular verbs etc. Then I also had a short chat in Spanish about my trip to Barcelona which I'd mentioned in my personal statement - felt weird but it wasn't anything to worry about, only lasted about a minute.
Really thinking about why I wanted to study my subjects: asking myself why I was interested in language, why I wanted to continue with Spanish, how Portuguese culture entered onto my radar… It was also useful to read some books (anything general about language, like "the unfolding of language")/watch some tv/listen to some songs, and develop some thoughts about them, so you have something to talk about and relate ideas to.
Be yourself!