blue_inversions, Oxford is on Easter break until April 7 or 8, I believe. They are delayed due to some larger changes in Graduate Admissions (and also, uh, it's Oxford and nothing runs on time), so if you applied for the March deadline, I don't know when you can expect to hear back, but if you applied for the November or January deadlines, it is wholly reasonable to email the department directly, explain the other offers and deadlines you are working with, and ask for an update. They're well aware that their timetable is often at odds with those at other universities, particularly North American programs, so I can't imagine they'd be anything less than sympathetic.