Windows in several buildings had been blown out, and people were in the street bleeding, NRK said on its website. The cause of the blast remains unknown.
There are conflicting reports about whether a second blast followed the first, which occurred mid-afternoon in the center of the Norwegian capital.
One explosion happened near a government building housing the office of Prime Minister Jens Stoltenberg, said Linda Reinholdsen, a reporter for Norwegian state broadcaster NRK. Another hit near the Norwegian parliament, she told CNN.
Several buildings in Oslo were on fire, she said, and smoke was pouring from them.
Walter Gibbs, a journalist with Reuters, said he saw eight injured people, including two or three with serious wounds and one who looked dead.
Gibbs said he believes one explosion happened on an upper floor of a main government building. He said it blew out every window on the side of the building.
