Throughout the country, parkruns take place every Saturday morning at 0900. The parkruns are free timed 5K runs, but you need a barcode to take part. So if you've not done one before, you need to register at www.parkrun.org.uk's registration page, print off a barcode and bring the barcode to the parkrun. A barcode can be used at any of the parkruns.
Below there are links to web pages for parkruns taking place near Oxfordshire. Here is a link to a list of other parkruns.
Not at all connected with the parkruns is the Risborough run in the park. This is a free weekly 5K for runners of all ages and abilities that takes place at 0900 every Sunday in Wades Park, Stratton Road, Princes Risborough [HP27 9AX]. From March 22nd, these events have UKA licence 2015-14407. There are more details on the web, Facebook and twitter.
Here are the links to web pages for parkruns taking place near Oxfordshire.