The residency requirement for PhD candidates is six terms within 25 miles of Oxford. See: Section 2. Requirements for study
For part time research students, the requirements are as follows:
Students studying a research course on a part-time basis should attend for a minimum of 30 days each academic year. This requirement, which is included in the special regulations for each research course offered on a part-time basis, is intended to ensure that part-time research students engage with activities (for example seminars and training sessions) within their department or faculty or more widely in the University, as well as providing opportunities for face-to-face meetings with supervisors.
In reality, if you wanted to live abroad, you would have to get your supervisor to agree and take you on that basis. If there was no good reason to be abroad, that might be more difficult than the rules make it appear.
Kimberley Lear, MA Philosophy, Politics, and Economics, University of Oxford (1996)
Source: Quora Digest