I have to take my son to Plymouth for a University course open day soon my route takes me past Oxford so I might have time for a detour/lunch stop!.... depending on how much of a detour it is.
If you are doing it on the way down, go Milton Keynes, Buckingham, Tingewick, Deddington, Chipping Norton, Stow on the Wold. Take the A429 (for Cirencester) in about 5 miles turn left to Bourton.
If on the way back, go to Cirencester, take the A429 to Stow and about five miles before Stow turn right to Bourton. Cotswold scenery from Cirencester to Chipping Norton and vice versa. Enjoy it.