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