Both of the Oxford-London buses offer a discount for those with Y-P or Student cards - the 12-trip ticket at £60 is a good buy. IME the X90 is faster and more reliable than the Tube, but not as frequent (also it's not Stagecoach, if you find Souter's activities distasteful). The airport buses and X5 also give discounts.
Fortunately OUSU has finally joined NUS so she will be eligible for a NUS card and can benefit from the discounts suggested by
unlevel42. I don't know what the discount is on NX, but she can also get a
Coachcard which gives a third off for £10 a year. Oxford has decent NX connexions but the train is usually quicker of course.
My experience is that most students default to the Tube/X90 over the train, although the train is often faster even after adding in the walk to the station. A day Travelcard is about £19 so comparable to bus+Oyster.
Congrats due and good luck with the exams!