A car was attempting to overtake a lorry at the junction of Blackberry Lane.
A 49-year-old man was taken to the John Radcliffe Hospital after the smash yesterday afternoon, where he remains at this time.
Police say two vehicles were involved.
A Honda Civic attempted to overtake a HGV at the junction, at the same time that the lorry was turning into Blackberry Lane on Watlington Road, causing a collision.
The Civic overturned and slid along the road.
Investigating officer PC Christopher Weatherley of the Roads Policing Unit, based at Abingdon, said: “This was a two-vehicle collision and I am appealing to anybody who witnessed the incident or who has any information to please contact Thames Valley Police.
“I’m particularly keen to hear from anyone who may have dash-cam footage from the area at around 3.20pm yesterday to please check this and contact us if it’s captured something that could assist the investigation.
"Likewise, if anyone saw a black Honda Civic or a blue/red HGV prior to the crash – this could be helpful to the investigation."
You can call 101, quoting reference 43210156840, or you can make a report online.