A 38-year-old man, who was a back seat car passenger, is in a critical but stable condition in hospital following the B9080 crash at about 22:15 on Wednesday.
The injured passenger suffered serious stomach and abdomen injuries.
The Vauxhall Zafira was travelling south when it crashed with a First bus between Kirkliston and Winchburgh.
The driver of the car, a 58-year-old man, suffered spinal injuries and a fractured ankle.
The front seat passenger, a 34-year-old man, had injuries to his face and leg. All were admitted to hospital.
Three of the bus passengers were treated in hospital for minor injuries. They have all since been discharged.
Police said the driver and six other passengers on the single-decker bus were treated at the scene.
The bus had 26 people on board.
A First Bluebird spokeswoman: "We can confirm that one of our Service 38 vehicles was involved in a collision yesterday at around 22:15 between Kirkliston and Winchburgh. The bus was travelling to Falkirk from Edinburgh.
"Unfortunately the incident resulted in a number of injuries and our thoughts are with the injured at this time.
"We have launched an investigation to establish exactly what happened and are assisting Police Scotland with their inquiries."
Police are appealing for witnesses following a head-on crash between a car and a bus in West Lothian.