Oslo was destroyed in fire in 1624 , and the fortress was rebuilt closer to Akershus Fortress by the order of King Christian IV of Norway . The king gave the new city the name Christiania , which later also written as Kristiania . Oslo was destroyed several times by fire , and after the fourteenth calamity , in 1624 , King Christian IV of Denmark ( and Norway ) ordered it rebuilt at a new site across the bay , near Akershus Fortress and given the name Christiania .