He was in private practice in Grand Forks from 1912 to 1925, and in Los Angeles, California from 1925 to 1933.

He served as Comptroller of the Currency in the United States Department of the Treasury from May 11, 1933 to April 16, 1938.

[4][5] In the 1920 gubernatorial election, O'Connor ran unsuccessfully against incumbent Lynn Frazier.

[6] O'Connor was nominated by President Franklin D. Roosevelt on August 28, 1940, to a seat on the United States District Court for the Southern District of California vacated by Judge William P. James.

His service terminated on September 28, 1949, due to his death in Los Angeles.