Got it! With a bit of help from an old Good Beer Guide and not much help from an ageing memory, the place I was trying to recall was Deddington, which is about six miles south of Banbury. The pub was called the Crown and Tuns, which appears to still be operative as a Hook Norton pub.
One of the odd things, in a Hook Norton sense, was that it wasn't a beer you saw very often in Oxford (that is, the city of Oxford). Hook Norton had no pubs in the city and the nearest, as I recall, was one in Woodstock, just near Blenheim Palace. The tied-house system was just ending when I moved from Oxford and you could get Hooky beer in the odd pub by then. One of life's many small ironies is that I have drunk much more Hook Norton beer since I've lived in Scotland than I ever did when I lived in Oxford. They have a very good web shop and pretty reasonable prices on their beer, so we are fairly regular customers between our various exploratory missions.