What is the best way to show that I have knowledge?

With regards to the Oxford interview, the tutors want to see your knowledge, of course. More important, however, is your attitude and your ability to think on the spot. They want to accept students who are interested (hence your knowledge), students who will work hard (exhibited by the fact that you have built up a bank of knowledge) and who are interesting and smart individuals themselves. They do not excpet you to be a genius - but they want to be interested by you. 

The best way, therefore, to show you have knowledge, is to listen to the questions they ask, to formulate an answer that may incorporate some of your own knowledge, and to show your process of thought. They will get you out of your comfort zone - being able to adapt is important. They do not want pre-prepared answers.