Oscar Mitchell was an accomplished and prominent attorney in Duluth and was senior member of the law firm of Mitchell, Gillette, Nye & Harris. In 1891 he was admitted to the practice of law in Minnesota and opened offices in Duluth. As a civic leader he was involved as the director and counsel of the Stone-Ordean-Wells Co.; director and general counsel of the Minnesota Power & Light Co.; organizer of the Great Northern Power Co.; director and general counsel of the Duluth-Superior Transit Co.; trustee of Carleton College, Northfield; Chairman of the Board of Directors of Trusts of Duluth. From 1900-1904 he was Duluth City Attorney. He also served as director of the Duluth YMCA.