Inside the stolen ambulance, police found an Uzi submachine gun, a shotgun and what they said were "large amounts" of narcotics. Police officers had fired shots to arrest the suspect, who was not seriously injured.
Police arrested the man after firing on the vehicle.
"It is too early to say whether this is terror-related, but we are investigating broadly and fully", police said in a statement.
Norwegian broadcaster NRK said Tuesday that several people were hit, including a baby in a stroller who was taken to a hospital. "They are twins, seven months old, they are being treated", Oslo University hospital spokesman Anders Bayer told Reuters.
Officers said those struck by the ambulance included a woman with a pushchair and an elderly couple. Some minutes later one of our other ambulances managed to stop the hijacked vehicle by crashing into it.
"They were called to an ordinary traffic accident where they were met by the armed person", Bayer said, adding that the hospital had quickly tracked the ambulance via its built-in Global Positioning System device.
The ambulance then crashed into a building, after which the police smashed the windscreen and pulled a man from inside.
One witness, Omar Khatujev, told the newspaper that he saw "the ambulance driving at high speed towards me. and right behind a police vehicle".
Television images showed a heavily damaged Mercedes ambulance with what appeared to be several bullet holes in the front left door.
A number of people were hit by the ambulance, police told Reuters. "He is not in critical condition". "It was crying", she said, adding that a woman and the baby were driven to hospital in police cars.
Police would not confirm if it was a deliberate attack, or if anyone had been killed.