Definitely take the bus rather than the train it is much quicker and cheaper. It is £20 for a period return not sure what the same day return price is but it is probably a couple of pounds cheaper.

If time is tight ask your friend to meet you in Headington which is the first suburb of Oxford the bus enters and that will save you time as the bus works its way into the centre of town. Although if you have time you will find a much better choice of restaurants in the centre.