Oscar J. Falnes was born on January 24, 1898, in Wood Lake, Minnesota. He received his A.B. from St. Olaf College in 1921, an M.A. from Stanford University in 1922, and a Ph.D from Columbia University in 1933. In 1927 he joined the faculty of the history department of New York University's Washington Square College. His research specialty was the intellectual and cultural history of Scandinavia, with particular emphasis on the movements of Romanticism, Nationalism, and Imperialism.

In 1941 he took a leave of absence from NYU to join the Office of Strategic Services (OSS) as a coordinator of information, later becoming a senior analyst and head of the Scandinavian-Baltic Research Intelligence Unit. In 1943 he was appointed to the Library of Congress as an associate fellow of the Library on contemporary Scandinavia. In 1944 he joined the United Nations Relief and Rehabilitation Administration (UNRRA), where he eventually headed the fellowship program.

He returned to Washington Square College in 1947 and worked in the history department until his retirement as Professor Emeritus in 1966. During that time, he served on several planning and evaluation committees. From 1957 to 1960 he was the assistant chairman of the history department.

During his academic career, Falnes contributed numerous articles to journals, periodicals, and encyclopedias, including the Dictionary of American History and Grolier's Encyclopedia International. He published two books: National Romanticism in Norway (1935) and Norway and the Nobel Peace Prize (1938). Falnes was an active member of various professional societies, including the American-Scandinavian Foundation, the American Association of University Professors and the American Historical Association, of which he served as the program chairman for the 1957 annual meeting.

Oscar Falnes was an avid traveler, taking his car on cross-country trips, and in the process collecting a large number of postcards from the places he visited. He died October 13, 1989, in New York.