On our last two visits to Oxford we've stayed at The Old Parsonage Hotel, and as you'll see if you follow that link, I have nothing but good to say of it. It is currently closed for refurbishment and the creation of five additional bedrooms and a rooftop library (!), and will be open again in March, so this time we stayed at its sister hotel The Old Bank. We had a very spacious and extremely comfortable room overlooking St. Mary's Church and All Souls, and its central location was ideal given that we were back and fore to Magdalen (a 5-minute walk away) several times over the weekend.
In the restaurant, the food was good and the service was slick, the staff throughout were very pleasant and helpful, and while the public rooms are not as cosy and almost cocoon-like as at The Old Parsonage, I'd still rush back any time.