The raw violence erupted around 6pm, when two men were shot repeatedly in broad daylight in the Kalbakken district of the capital on the city's northeast side.
One of the victims was also attacked with what police are calling a Samurai sword, in addition to being shot several times in the chest. Newspaper VG reported that one of the man's arms was chopped off, but police wouldn't confirm that.
Doctors at Ullevål Hospital in Oslo worked through the night trying to stabilize him and save his life. His condition remained critical on Monday.
A second victim was also reported in critical condition after suffering multiple gunshot wounds. A third victim was treated at Oslo's emergency clinic after being hit by what police believe was a car driven by the assailants.
There were at least 10 witnesses to the bloody shootout, including several young children, and police were scrambling to secure their accounts of events. Police said they found handguns, knives and the sword strewn over the scene around the football field at Nordvedt.
Police couldn't confirm what set off the violence, but said it apparently followed "an event" the day before, believed to be tied to a conflict within the Tamil community. All three victims are Tamils.