Police are appealing for Dashcam footage of four crashes involving more than 20 cars on the A40.
The pile-up happened at around 8.25am on Wednesday 28th December close to Witney.
A 56-year-old woman, who had been travelling in one of the vehicles involved in a collision, left her vehicle and was subsequently struck by another vehicle.
She was taken to the John Radcliffe Hospital with serious injuries where she remains in a critical condition.
Thames Valley Police are continuing to appeal for witnesses following multi-vehicle collisions on the A40, Witney, on Wednesday 28 December.
Officers were called at about 8.25am following reports of several collisions involving multiple vehicles on the eastbound carriageway.
A 56-year-old woman, who had been travelling in one of the vehicles involved in a collision, left her vehicle and was subsequently struck by another vehicle. She was taken to the John Radcliffe Hospital with serious injuries where she remains in a critical condition.
A white Peugeot was involved in a collision with a heavy goods vehicle between the B4477 and the A415 Witney. A 54-year-old woman from Gloucester, who was a passenger in the Peugeot, died at the scene. Her family has been informed and relatives are being supported by specially trained family liaison officers.
A number of people also sustained minor injuries and were taken to local hospitals, from where they have since been discharged.
The road was closed while officers and the other emergency services attended and re-opened later in the day.
Thames Valley Police officers remain at the scene of multi-vehicle collisions on the A40, Witney, which took place at around 8.20am today.
In a series of crashes this morning, a woman in her fifties, was killed and a man and a woman were taken to hospital with serious injuries.
The westbound carriageway of the A40 now has one lane open and the second lane is due to reopen once repairs have been completed to the central reservation.
The eastbound carriageway of the A40 will remain closed for a further three hours to allow the last vehicles to be recovered and the road surface to be cleared of debris.
An Oxfordshire driver has captured the aftermath of some of today's A40 crashes on his dashcam.
Chris Seymour, 39, who lives near Burford, said: "The road conditions were horrendous with minimal visibility."
Footage shows the scene of a major crash and a number of minor shunts in the lay-bys and to the side of the road.
Another eyewitness arrived at the scene later and described the "carnage" as multiple cars swerved and crashed on the busy road.
The woman, who asked not to be named, said she "luckily" saw hazard warning lights ahead as she approached on the slip road from Carterton, and moved on to a grass verge before witnessing the pile-up.
"I saw cars going zooming past me and trying to swerve out the way of the stationary cars but crashing into them," she said.
"I saw one car try swerve in between a Mini and another car, ending up crashing into the Mini and (flying) up in the air.
"I saw two cars swerve as they braked so hard and go flying into the barrier in the middle of the road ... it was carnage," she added.
A pregnant mother from Oxfordshire who was a passenger in a car on the A40 has described the moment she saw the first vehicle crash.
Ami Gaston, from Carterton, says she was in the car with her partner when she saw the accident happen about half a mile from the Witney exit, blocking the right lane.
She said: "A man on his own had skidded and crashed into middle barrier. My partner was driving - we had to swerve out of the right lane to avoid hitting him. His door was open and air bags were released," Miss Gaston said.
"I phoned an ambulance but couldn't stop. My partner wouldn't stop because I am 20 weeks pregnant and didn't want to put us in danger."
She added that the heavy fog meant the situation was "really dangerous", as it was difficult to see any other car until you were "on top of it".
Drivers are being urged to avoid the A40 following a multi-vehicle collision.
A woman has died and numerous injuries have been reported. Up to twenty vehicles were involved in the crash on the A40 in Oxfordshire.
More details via ITV National News.
This coming in from Oxfordshire County Council:
Motorists are being advised to take alternative routes to avoid the A40 between the B4047 Asthall Roundabout and A415 Witney today as the road is currently shut following a multi-vehicle accident.
Oxfordshire Fire and Rescue Service, police and ambulance services are in attendance.
It is expected that the A40 between the B4047 Asthall Roundabout and A415 Witney will be closed for most of today, and this will have a knock-on effect on traffic on the rest of the local road network.
More to follow.
A40 Westbound closed, queueing traffic due to serious accident, a car and a lorry involved between B4022 (Witney East) and A415 Ducklington Lane (Witney Town Centre / Ducklington Turn Off).
If you don't want to speak directly to the police you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online.