Having recovered from his wound, he returned to the north to participate in the war. Otto embarked on the auxiliary cruiser "Amazonas" as commander of the military garrison on board. In October, during the naval campaign, he had the advantage of being one of the legal participants in the price of the capture of the Peruvian ironclad "Húascar". Later Otto was reinstated to the Marine Artillery Regiment with the rank of lieutenant. He participated in the Battle of Pisagua and the Battle of Tarapacá, both in November, during the Campaign of Tarapacá, and the Battle of Tacna in May 1880, during the Campaign of Tacna–Arica. During the course of the latter, his regiment was one of the military units that decided the Chilean victory in the center of the battle line.