Prior to his sheriff's tenure, Haynes directed the Louisiana state driver's license office in Minden, a position which gave him contact with voters who would later elect him four times as sheriff. In Louisiana, the sheriff is the collector of property taxes and also enforces criminal laws outside the municipalities.
Haynes entered the December 7, 1963, Democratic primary against incumbent Sheriff John D. "J.D." Batton (1911-1981) and several other contenders, including Royce L. McMahen (1923-1999), a veterinarian and a member of the Springhill City Council. Haynes trailed Batton by 722 votes in the primary. McMahen, who ran a strong third in the primary, endorsed Haynes in the runoff. Haynes then announced that McMahen would be his chief deputy. In his runoff campaign, Haynes ran a newspaper advertisement in which he vowed to bring "capable, conscientious, and sober leadership" to the sheriff's department. He claimed that the issue was not one of physicial equipment or the training of deputies but the leadership skills of the individual chosen as sheriff. In the January 11 runoff, Haynes prevailed, 5,190 votes (53.4 percent) to Batton's 4,523 (46.6 percent).
Haynes appointed the African American deputy Louis Dunbar, Sr. (1914-1986), whose son "Sweet" Lou Dunbar became, like Haynes' son, a distinguished athlete. Dunbar played for twenty-seven years for the Harlem Globetrotters.
In 1967, Haynes scored a second term by again defeating Batton and a second primary candidate named Red Vaughan. Haynes prevailed with 6,952 ballots (53.3 percent) to Batton's 5,456 (41.8 percent) and Vaughan's 634 (4.9 percent).
In the 1971 primary, Haynes defeated two opponents, including the Minden ward marshal, John T. Kennon, Jr. (1928-2005), a nephew of former Governor Robert F. Kennon. In the general election on February 1, 1972, Haynes obtained his third term by defeating George A. Pipes (1913-1976), a businessman from Dubberly in south Webster Parish and the only Republican candidate who ever opposed Haynes in the historically Democratic parish. Haynes polled 9,801 votes (81.9 pecent) to Pipes' 2,167 (18.1 percent). Pipes had also been one of the minor Democratic candidate for sheriff in 1963.
In his last election in the first-ever jungle primary in Louisiana in 1975, Haynes defeated Ward Marshal Johnny Lombardino (who had succeeded John T. Kennon in the position) and a third contender, Waymon Nealy, 8,344 votes (57.7 percent) to 5,905 ballots (40.5 percent), and 192 votes (1.8 percent), respectively.
Haynes did not seek a fifth term in 1980 but instead supported McMahen as his successor. Instead Haynes returned to private business. Haynes, who was Baptist, died at his residence after a lengthy illness. He is interred in the Haynes-Walker family plot in Minden Cemetery.
O.H. Haynes, III (born November 5, 1943), was employed in various capacities in the sheriff's office from 1973 until his retirement in 2008. His wife, Debbie Haynes, remains an administrative assistant to current Sheriff Gary Sexton. Their son, O.H. "Hank" Haynes, IV (born May 4, 1967), is employed as a Louisiana State Police trooper.