The light aircraft crashed into an uninhabited apartment block near Camp Road in Upper Heyford at noon.
According to eyewitnesses, the pilot was seen parachuting onto the roof of a former RAF base before the plane nose-dived and crashed into the apartment block.
The pilot was taken to John Radcliffe Hospital in Oxford, but the extent of his injuries is unknown.
Thames Valley Police said no one else was injured.
Tisa Kilby, 55, told how she was walking her dogs when she saw the plane crash into the building.
She told BBC: “I looked up and literally saw this guy jump out of this plane and thought it’s a weird place to parachute.
“Within four or five seconds, the plane dived with its nose and crashed into the apartment block.”
Miss Kilby said police at the scene told her the pilot had sustained life-altering injuries.
The South Central Ambulance Service, which described the victim as an “elderly gentleman”, said the man was treated on the spot before being taken to hospital.