Admissions to postgraduate programmes in law are managed on the Faculty's behalf by the University's central Graduate Admissions Office. The University's Graduate Studies Prospectus provides authoritative information on when and how to apply. The prospectus and the associated application forms are available online or as paper documents; applications should be made online, other than in exceptional circumstances.
Note that every Oxford graduate student must be accepted by a college as well as by the Faculty. Both Faculty and colleges make decisions on academic grounds using the same selection criteria. Once accepted by the Faculty you are guaranteed to receive an offer from a college - but not necessarily the college you chose on your application form.