At the OU
There is a 6-aside tournament that runs from March to May. This tends to be a departmental thing, so ask around to see if your department has a team. Teams range from the serious to the non-serious (the final for the Desmond Dribble Cup is played in fancy dress!).
The OU has a football team that play in a Sunday league for the serious!
Outside the OU
The MK Dons play at Stadium MK in Bletchleyfrom August to May each year.