Applying to Oxford and Balliol
If you want to apply as a graduate student, here are links to the University prospectus and to some information about Balliol. Some more specific information about philosophy of physics (and some more links) can be found on my site here . (Also see here if you are already a graduate student elsewhere and would like to visit Oxford.)
Undergraduate study in general
Undergraduate study in philosophy
Philosophy at Oxford is always studied as part of a joint degree course. The University prospectus provides more details; you might also want to look at Balliol's subject pages to find out more about a particular course at Balliol.
In my own teaching, I am particularly involved with three courses: physics and philosophy, maths and philosophy, computer science and philosophy; follow the links for more information on these courses in particular.