See also the OCC Oxfordshire Local View pages, specifically the "on the map" tab with Map Layers for displaying various boundaries (Parish, District, County, Parliamentary), Post Code Units and much else.
Outline map below is from the 2001 census site
It also appears that the Church of England Harwell Parish boundary matches the civic boundary, so this map also reflects the Parish boundary.
Web site and contents © 2005-2013 Harwell Parish Council, PO Box 223, Wantage OX12 2DH, Oxfordshire, England.
The book "Harwell ~ Village for a thousand years" © Harwell Parish Council, published 1985, ISBN 0 9510668 0 3