How should I prepare for an application for Oxford?

People applying to Oxford have a wide range of levels of preparation so first of all you don't need to worry about not having read enough or feeling inadequate compared to other people applying! My advice would simply to be read as many books about the subject you're interested in as you can, making sure that you read regularly on topics outside of your ordinary A Level or IB curriculum, There is no right or wrong answer in terms of what is a good topic to read so just follow your passions- tutors in the interviews are above all looking to check that you have a genuine interest in your topic and you can talk about it eloquently and engage with ideas that you may not have heard previously. This doesn't mean that you have to be an expert! No one is expecting you to be the finished product when you do your aptitude test or your interview, you simply have to show a willingness to adapt and engage with concepts that are new to you.

Have a look at past papers for the HAT (or whichever Aptitude test is relevant to your chosen course) and have a think about the questions there- maybe try and answer some of them (especially the comprehension-style ones) but don't worry if they're unlike anything you've done before and you think you haven't answered them properly- that's the point! It won't be marked like a normal essay so just be creative and try to think outside the box, linking ideas from various topics you have covered to answer the question in a unique way.