In addition to practicing law, O'Connor engaged in stock raising, ranching, and banking. He served as judge of Montana's sixth judicial district in 1912. He served in the Montana House of Representatives from 1917 to 1918 and was the Speaker of the House.
O'Connor served as special counsel for the Federal Trade Commission in Washington, D.C., in 1918. He also served as member of Park County High School Board for a number of years. Following the death of United States Senator Thomas J. Walsh, O'Connor ran for the Democratic nomination to succeed Walsh in the 1934 special election, challenging James E. Murray and John E. Erickson, who, as Governor of Montana, had appointed himself to the seat. O'Connor lost to Murray, but finished ahead of Erickson. Murray went on to win the general election.
In 1936 incumbent United States Congressman Roy E. Ayers ran for Governor and O'Connor ran to succeed him in Montana's 2nd congressional district. O'Connor won the Democratic primary and defeated T. S. Stockdahl, the Republican nominee, in a landslide. O'Connor was narrowly re-elected in 1938 over W. C. Husband when Republicans nationwide gained eighty-one House seats. He won re-election in 1940 in a landslide against Melvin N. Hoiness, and again in 1942 against F. F. Haynes by a much narrower margin. O'Connor was an isolationist during the buildup to World War II, and he voted against providing aid to Britain in 1941.[2]
O'Connor died in Washington, D.C., on January 15, 1945, just weeks after beginning his fifth term. He was interred in Mount Calvary Cemetery, Livingston, Montana.