Is a day trip to Oxford from London reasonable, or should I plan an overnight?

Question

I am planning a Christmas week trip there (from the States) and want to take my niece to some Harry Potter sites in Oxford

Answer 1

Yeah it should be easy, in addition to the bus there are regular trains, the journey from London should only take about an hour.  Make sure you book in advance though (use the national rail website), the earlier you book, the less you pay.