To follow up my own post, I thought of two other candidates for the central area: * the OX1 postcode area: http://maps.google.co.uk/maps?q=OX1 * Carfax & Holywell council wards: http://www.oxford.gov.uk/Direct/SOAProfilesHolywellandCarfaxWards.pdf as well as the city walls: http://www.headington.org.uk/oxon/city_wall/
The OX1 postcode has a useful northern boundary including the University Parks but excluding Jericho. It also includes the railway station. To the south, it's less use, as it includes all of Abingdon Road and even Boars Hill.
The two central council wards together exclude the railway station; the River Cherwell is an eastern boundary and the railway line part of the western boundary.
Owen