Following a United States entry into the Korean War in June 1950, he was assigned to the newly activated "G" Company, 3rd Battalion, 1st Marine Regiment at Camp Pendleton, California and sailed for Korea at the end of August that year. Olmstead was promoted to the rank of Private First Class and participated in the Inchon Landing in September 1950 and then in Chosin Reservoir Campaign in November–December 1950, where he served as Squad leader.