Billy O'Brien (baseball)

William "Billy" O'Brien (born August 23, 1939) is a former professional baseball pitcher. He played in Major League Baseball (MLB) from 1962 to 1968 for the St. Louis Cardinals, Baltimore Orioles, and Atlanta Braves.

O'Brien was signed as an amateur free agent by the St. Louis Cardinals in 1958. He made his major league debut on April 11, 1962. Primarily a relief pitcher, O'Brien also started games occasionally. His best season was arguably 1964 with the Cardinals, where he appeared in 48 games and had a 3.31 ERA.

O'Brien was traded to the Baltimore Orioles in 1965. He was later traded to the Atlanta Braves in 1967, where he finished his major league career in 1968. After his playing career, O'Brien was involved in coaching and scouting.