The incident occurred this morning (Monday, November 4) on Banbury Road in Summertown.
A serious crash involving a moped and a bicycle and closed the road in both directions between Oakthorpe Road and Beechcroft Road.
The cyclist has been taken to hospital with serious injuries.
Police have now warned the public to stay away from the area as their investigations are likely to continue for quite some time today.
A spokesperson for Thames Valley Police said: “The road is likely to remain closed for several hours while investigations are underway.
“We would ask the public to please avoid the area and find alternative routes while emergency services are dealing with the incident.”