Seventeen people were hurt when a coach veered off a motorway slip road and overturned while driving in heavy fog.
The Oxford Bus Company vehicle came off the M40 northbound slip road at junction seven near Thame at 02:45 GMT.
South Central Ambulance Service (SCAS) said a variety of injuries had been suffered, including cuts, bruises and broken bones.
Phil Southall, managing director of the bus company, said it was too early to speculate on the cause.
He said the 54-year-old driver was “very experienced” and had been at the company for 20 years.
Mr Southall said: “We are thankful that there were no serious injuries and we have been advised that injuries incurred were a spinal injury, a broken collarbone and minor head injuries.
“Our thoughts are with those who have been injured, particularly at this time of year.
“Company representatives have spoken with all those involved and are continuing to assist the emergency services with their investigations.”
The bus was returning to Oxford from Heathrow airport at the time of the crash.
Thames Valley Police said the slip road will be closed for several hours while recovery work takes place.
Sgt Mark Scully said: “It would have been a chaotic scene, not aided by the dense fog, the time of night, and the cold as well.”
He said officers was considering two lines of inquiry – one being the “very hazardous driving conditions” and the other being the fact the bus was leaving the motorway at junction seven, instead of junction eight towards Oxford.
A spokesman for SCAS said weather conditions had presented challenges to the emergency services getting to the scene.
He added: “The male coach driver and 16 adult passengers (a mixture of male and female) had sustained a variety of injuries, such as cuts, bruises and broken bones.
“All 17 patients were taken to the John Radcliffe Hospital in Oxford.
“At the scene we had six ambulances, a rapid response vehicle, three ambulance officers and the trust’s hazardous area response team.”
Mick Clarke from Oxfordshire Fire and Rescue Service said: “Seventeen casualties have luckily been removed.
“The investigation is ongoing and is squarely in the hands of Thames Valley Police.”
A yellow fog warning has been issued for much of southern England with the Met Office saying “freezing fog” could result in poor visibility until the afternoon.
Mr Clarke added: “The M40 goes through several altitude changes where you dip into the valleys and up again and some areas can be more prevalent to fog, so you can be driving along nice and clear at one point and then hit steady fog banks.
“We’ve got some really tricky weather conditions this week, it has been quite changeable.
“We have had real thick patches of fog in different areas with freezing temperatures which lead to these slippery conditions.”
A similar crash happened near the same spot six years ago in which 17 people were also taken to hospital when an Oxford Tube bus overturned.
A woman died and several other people were injured in four separate crashes involving more than 20 cars on the A40 in Oxfordshire on Wednesday.
Read more at BBC.co.uk