Have you ever looked at a map of North Summertown and wondered why Wentworth Road turns into Aldrich Road and Carlton Road turns into Wolsey Road? The reason is quite shocking!
Back in the mid 20th century, these roads were separated by walls - 7 feet high and topped with metal spikes. They were put up in December 1934 by a housing company, to prevent the residents of the adjacent council estate from walking through their (private) development. They were eventually knocked down in March 1959 after many years of campaigns and legal challenges. You can find more info at http://www.oxfordshireblueplaques.org.uk/plaques/cutteslowe.html There is also a really interesting Oxford Times article called "And the walls came tumbling down" available on the internet, but I can't include the link here as there are adverts in it.
The first house in Aldrich Road has a blue plaque to mark the position of the southern wall. The cache is placed a short distance away, where it is hopefully less likely to be muggled. You are looking for a small key container (reinforced with extra magnets!). PLEASE BE ULTRA STEALTHY AND REPLACE EXACTLY AS FOUND.
Please note that there are severe parking restrictions in all the nearby roads during the daytime on weekdays, so (unless you are coming here by bike or on foot) you might prefer to come after about 5.30/6.00pm or at the weekend.