What is the cheapest way commuting between Oxford and Plymouth [and vice versa] as a student?

Depends on how often you're planning to do it. Daily, totally impractical, even with a car (�50 a day) , plus it would make a 7 hour travelling hole in your day. With fuel costs you could rent a room in Plymouth and have change left over. Beginning and end of term, then train or bus but both are in excess of �30 even with a railcard or a bus card and take forever.