Luggage, Baggage, Suitcases. You can leave them somewhere safe and convenient…
There are many options is Oxford.
At the Station, at the Ashmolean. This is a good site to find the best fit for you.
There are also left luggage lockers at the Ashmolean Musuem, some big enough for all but the largest suitcase, but remember when the Ashmolean closes you will not be able to get to your locker.