I've debated this on TSR inside out many times before, and the things to know are:
1) Oxford terms are 2 weeks shorter than other universities. A full-time job in those 2 weeks will earn you more money than a shift per week during term.
2) If you genuinely have financial difficulties, there are generally college funds available to help you out.
3) If you still insist on working, then job regulations are college specific. Most colleges, as far as I know, have a rule that part-time work may be taken up with permission of your tutor. This is to ensure that your academic work is not affected by any financial problems you may be having.
4) If you still really want a job, and you're doing OK academcially, I doubt your tutor would mind you doing a shift or two a week. But bear in mind that they will want to see that you can cope first, so maybe you wouldn't get their permission in your first term at Oxford.
5) However, despite all this, the strongest argument I've seen for getting a job if you want one is that some people spend 20+ hours a week on rowing/the Oxford Union/being JCR president. If you want to spend 8 hours a week working for a bit of cash/CV points, then I can't see a problem if others are spending an equal amount of time doing something with a similar time commitment. And it has been known for people to be banned from rowing by their tutors for not performing properly academcially, but this is generally only if you are a complete bum.