Luggage storage in Oxford

When looking to leave luggage somewhere for a few hours in the UK, for larger places my normal go-to is one of the commercial left luggage places at a large station, though sadly Oxford isn't one of the places that offer it. Another good option is to leave it where you've just stayed / will be staying, assuming that's the same city that is!

This week, a friend is going to be passing through Oxford for a few hours, and sadly they've timed it for when I'm not there to look after their bags. They're passing through Oxford on their way elsewhere, so don't have accommodation in the city either before or after where bags could be left. They're planning to do a fair bit of walking around the city, which is much less fun dragging a suitcase.

Is there anywhere in Oxford that they would be able to leave their luggage for a few hours? And how much does it cost for a small-ish suitcase?