Ole Østmo was born in Elverum, but moved to Oslo in his early 20s and was a member of Christiania Skytterlag his whole career. At the first World Shooting Championships in Lyon, France, in 1897, he won the free rifle, standing competition, won a silver medal with the Norwegian free rifle team, placed second in the three positions event, and was third in free rifle, prone. Three years later he was back in France for the rifle events in the Paris Olympics, winning two silver and two bronze medals. Later he competed very rarely, and the 1900 Games was his only Olympic appearance. Østmo belonged to the so called Gressholmen Team, a group of shooters who used to train at Gressholmen, a small island in the Oslo fjord.