Somerville admits undergraduates to read almost all the Honour Schools which include Philosophy: PPE (Philosophy, Politics and Economics), Lit Hum (Classics), Psychology and Philosophy (PPP), Philosophy and Modern Languages, Mathematics and Philosophy, Physics and Philosophy, (but not Philosophy and Theology).
Note that it is not possible to study Philosophy on its own at Oxford. Full details of particular courses can be found under the appropriate headings, and on the Faculty of Philosophy website.
There are usually about fifty undergraduates at Somerville studying Philosophy as part of their course; they form a large and lively group. In all the courses it is possible to choose to weight your studies in favour of one or the other subject, or to divide your time equally among them. No background in Philosophy is required, and very few applicants have made any formal study of Philosophy.
An Introduction to Western Philosophy by Antony Flew (Thames and Hudson, paperback) contains extensive selections from major philosophers, with linking commentary and an overview. Think by Simon Blackburn (Oxford University Press 1999) is an excellent introduction. Reading one of these may help you to find out what Philosophy is like and whether it appeals to you, but such reading is not required as preparation for the admissions procedures.
To study Philosophy you need to have a flexible, critical intellect and to be willing to study a wide range of abstract questions. Though Philosophy is an 'Arts' subject, it can be enjoyed by those from a variety of disciplines - science, maths, languages, other arts subjects, the social sciences. You will read the works of great philosophers written over a two thousand year period, as well as the latest writings on the subjects, and will be encouraged to evaluate philosophical arguments for yourself.
In all the courses, you will study some History of Philosophy; Ethics (moral philosophy) is an option in all schools and compulsory for some. There is a very wide range of further papers, including Philosophy of Mind, of Science and of Religion. Aristotle, Kant and Wittgenstein can be studied in special papers; you will also encounter their writings in some of the core subjects.