I stayed in a lovely one in February, the room isn't cheap and breakfast isn't included but the attached restauraunt is good value for money. Very close to walk everywhere, just opposite the Bodlian Library www.oldbank-hotel.co.uk/
Yup, was about to say the same. We always book via laterooms and get a decent price. Breakfast is indeed expensive but if you go for the whole buffet and cooked combo, sets you up for the day It's pretty much the only central one to have a car park - others do but are a longish walk into town.
The old parsonage - not quite as central as the old bank but much nicer and only a 5 min walk to the centre (oxford is v small, all v walkable) and near lots of nice restaurants (on little clarendon st and walton st - nicer area for eating/going out than the centre of oxford)