I talked generally about my reasons for wanting to study in Oxford - both my career goals and my academic interests. I then mentioned the specific modules that I thought would help me achieve my goals/further my interests and explained how my previous experience and study would allow me to contribute to class discussions, etc.
Sorry that that's very vague. I'm sure it differs for each course. I was applying for a law course, so the career/academia link was very obvious.
Best of luck!