After Romania entered World War I in August 1916, Olteanu was put in command of the 2nd "Călărași" Regiment, and then served as Chief of Staff of the First Army Corps. He was promoted to brigadier general in 1917, and became commanding officer of the 3rd "Roșiori" Brigade. He distinguished himself at the Third Battle of Oituz in July–August 1917, especially at the engagement at against German forces. (His son, Marcel M. Olteanu, a graduate of the Dealu Monastery military school, also fought in this battle.) For his valor Olteanu was awarded in September 1920 the Order of Michael the Brave, 3rd class. In early December 1917, he was put in charge of the central command of the Romanian Volunteer Corps in Russia, based in Hârlău, Iași County.