Oscar Lunseth was born August 14, 1894, in Grand Forks, North Dakota. He was the son of John and Caroline (Olsen) Lunseth. His father came to Grand Forks in 1887, and served as the City Superintendent of Water Works. Oscar Lunseth attended public schools in Grand Forks.
Lunseth joined the United States Navy during World War I, enlisting at Grand Forks December 10, 1917. He served at the Great Lakes Naval Training Station in Illinois until his discharge May 6, 1919. During his time in the Navy, he served as a Fireman Third Class, Plumber and Fitter, Water Tender and Chief Water Tender.
Following the war, Lunseth returned to Grand Forks and took a position with Dakota Plumbing and Heating. He first began working for this company in 1912. He worked for Dakota Plumbing and Heating for over twenty-three years, leaving in 1935. In July of that year, Oscar and his brother Rasmus founded Lunseth Plumbing and Heating. Located at 135 South Third Street, the firm specialized in "boilers-stokers-oil burners-air conditioners-water softeners-farm water systems." Oscar Lunseth served as vice president and secretary of the company until his death in 1976.
Lunseth was active in Grand Forks city government, serving as alderman from 1940 to 1952, and mayor from 1952 to 1960. On a statewide level, he was especially concerned about the issue of water conservation. He was a member of the North Dakota State Water Conservation Commission (1951-1965), and served as director of the Missouri-Souris Projects Association. He also served on the Red River Basin Planning Committee and the North Dakota Reclamation Association. He was honored by the Sertoma Club in 1975 with the "Service to Mankind Award." He was chosen for the award due to "his pioneering work in the field of water conservation and purity." He was a member of the American Legion, the Sons of Norway, the Elks and United Lutheran Church. He was active in the State Association of Master Plumbers of North Dakota, serving as both president and secretary.
He married Sophia Bergetta Bentley on December 31, 1921, in Twin Valley, Minnesota. The couple had three children: John, Lois and Louanna. Oscar Lunseth died February 23, 1976, in Grand Forks.