The single-decker coach and the red Renault hatchback car crashed on a sharp bend, a notorious local black spot, on the A424 road at the junction with Donnington Trout Farm, near Moreton-in-Marsh at 3.30pm.
The pensioners, including some in wheelchairs, were described as "badly shaken". They are understood to have been visiting the Cotswolds on a trip from Telford in Shropshire.
They were initially taken to the local village hall where several were treated for minor injuries and shock. A number were then taken to Cheltenham General Hospital by ambulance although none were believed to be seriously hurt.
An air ambulance took the fifth car passenger to the hospital, where his condition was described as serious. He was cut from the wreckage of the Renault by firefighters, along with the four deceased. All the men in the car are believed to be from the Birmingham area.
The accident was witnessed by Phil Janka, 38, a brewery worker who lives near by. He said: "The car was going towards Stow-on-the-Wold from Evesham when it met up with the coach going the other way and hit it head on.
"The impact has pushed the car back quite a distance and it has been squashed to about half its size."
He added: "This is a nasty bend and there have been a lot of accidents on it before, but never anything this serious."
The cause of the accident was being investigated by police.