In 1952 he chaired the Adlai Stevenson's presidential campaign in his Iowa congressional district.

[5] In 1961 President John F. Kennedy appointed him the United States Attorney for the Northern District of Iowa, a position he held until 1967.

Soon after leaving office, he became an advance man in the 1968 presidential campaigns of Robert F. Kennedy,[6] then George McGovern,[7] and later Hubert H.

[9] In 1977 he served as special counsel to a subcommittee of the United States House Committee on Small Business for its investigation of anti-competitive practices in the meat industry.

On September 27, 1978 Carter nominated O'Brien to succeed Judge William C. Hanson in the third of those positions.