The battle of Oslo took place on 21 April 1240 between King Håkon Håkonsson, Duke Skule Bårdsson and their warriors. The king and his leidang (from Western Norway) sailed with several ships from Hovedøya in connection with the attack on the duke in Oslo.

The king's forces landed at Eikabergstøa below the Ekeberg slope, and took the town from the south via the partially destroyed Geitabridge.^[1] The Vårbelgers under Duke Skule were chased fleeing into the Hallvard Cathedral or out of the city. Many were killed outside the Hallvard Cathedral, but inside the church the Truce of God reigned and the refugees could not be persecuted and killed there. Hertug Skule fled north, but was killed outside Helgeseter monastery in Nidaros a month later.^[1][2]

In the spring of 1240, after the Battle of Låke and the capture of Valdisholm castle north of Sarpsborg in April, Skule had conquered Eastern Norway. King Haakon sailed to Eastern Norway with his forces, which probably numbered between ten and forty ships. Due to winter, the leidang in Western Norway were perhaps prevented from participating in the campaign eastward, and several sea voyages in February 1240.^[2]

Earl Knut Haakonsson, who had experienced a sense of defeat, on the king's order gathered all remaining forces and awaited the king's arrival. At Jersøy, the two army forces were gathered on 19 April 1240 and from there the king's fleet continued up the Oslofjord.^[3] He later got to keep his title as earl due to his loyalty to Haakon IV, and became the third last to recieve this title in Norway.^[3]

The ships arrived at Nesodden on April 10. The king hoped to make a surprise attack on Oslo regardless of whether the Vårbelgers were there or not. The army was to depart from Hovedøya and attack Oslo from three sides: at the bridge over the Akerselva at "Nedre Foss", at the piers and at "Ekebergskråninga" where the king himself was to disembark with his supposedly strongest force to take the city from the south.^[1][2]

Skule sent detachments to secure the flanks and ordered Hallvard Stutt to tear down Geitabru which is the bridge over the Alna River to prevent access from the south. The Vårbelger forces were in the process of removing the wooden planks from the bridge, but they withdrew when they saw enemy forces on the other side of the river. The advance down Korsbrekka had been delayed by the soft clay. The king and his men crossed the bridge and formed ranks on the riverbank before continuing into the city. At Østre strete, the two shield walls clashed outside Asbjørn Kopp’s farm near Hallvardskirkegården.

The flanking attack from the south took the Vårbelg forces by surprise. They defended themselves with formations of archers and stone throwers, halting the Birkebeiner advance on both Østre strete and Vestre strete with a barrage of arrows and stones. Skule, mounted and accompanied by many men, was positioned at the upper Geilene near Korsbrødregården, northeast of Hallvardskirkegården, and was unprepared for this resistance. Sveithøvding Alv from Løvestad tried to stop the advance from Østre strete along the river but fell into the water and perished after defending himself as best he could. Leading his men, the king pushed further east and maneuvered around Hallvardskirkegården.

Initially, Skule intended to launch a counterattack along the river behind the cemetery, but his demoralized men refused to obey orders. He withdrew with his troops to Martestokker. Meanwhile, the royal forces were flooding into the city from the south and the docks, executing a pincer movement against Skule’s troops. At Martestokker, he reorganized his remaining forces and resumed the fight against the advancing Birkebeiner warriors.

However, the king entered Korsbrødregården and then moved north around Hallvardskirken. There, he had a skigard (palisade) broken down, allowing the Birkebeiner troops to storm into Hallvardskirkegården, which was surrounded by high and strong stone walls. Many of the Vårbelg fighters were unable to get into the church through the southern door due to the crowding. Over seventy men were killed in the churchyard.

The Vårbelg warriors who sought refuge inside Hallvardskirken did not trust that the Birkebeiners would respect the sanctity of the church. They barricaded the church doors to prevent them from being forced open. Only on the following day did the defeated Vårbelg forces surrender. The king gave his word that they would be granted quarter. It was Sunday, April 22, 1240.

Skule had been forced to flee the city to Martestokker, where he and his men fought the Birkebeiner forces. Realizing they were outmatched, they abandoned the fight. Most of the Vårbelg warriors either remained in the city or fled. With a company of ninety men, Skule retreated to Eidsvoll.

• Håkon Håkonssons saga

• Håkon Håkonsens saga