Seeing you have a car, I'd drive. It's two hours max. Plug Cambridge and Oxford into ViaMichelin and take that route rather than the M25/M40 other online maps give.

Be aware, parking in Oxford is as atrocious as Cambridge, so either use a Park & Ride service from the outskirts or if you do drive into the centre, park in the first place you see, else you'll go nuts searching (that 'maybe I'll try to get a little nearer' voice in your head.... BAD!!)

If you really don't want to drive, take the coach. Train is not an option in a sensible time.