All car cages took place in Stordamveien on Godlia, the police quotes.
The first message about the fire came to the police at 2.48 am. No one was arrested on Sunday morning.
"We have few observations from witnesses, there is nothing else that indicates anything else, but the cars were randomly chosen, says operations manager Christian Engeseth at NTB.
Two of the cars were on private runways and were close to homes. There were a total of four cars that were burned at three different locations in the area.
"It was clear to wait when so many cars start burning at the same time, we have looked for perpetrators in the area with helicopters and a lot of units, but without result.We have now towed the burned cars and we will continue with research and tactical research, says the operational manager on Sunday morning 4 hours.
The situation can be experienced as intimidating, with a large smoke development and all emergency departments on site.
"We did not evacuate anyone, but there was a lot of smoke development, so residents had to keep windows closed, from a car burn it spread to a garage, from another it spread to a hedge, but in the end the fire department got control of the fire" says operations manager Gjermund Stokkli.
He also says that there were a number of high explosions in the area, which should have been the sound of tires and roots that exploded because of the fires.