How much reading around your subject should you do?

It varies depending on the subject, but you should generally try and have two or three areas within your subject (not covered at school) that you're interested in and are able to talk about at reasonable length. However, the tutors aren't looking for someone who knows everything; they're looking for students with genuine enthusiasm for their subject who are well suited to the teaching style at Oxford. This means not being afraid to ask questions, exploring all possible solutions to problems you might be presented with, and (arguably most importantly) having an open mind.