Jack Whalen (May 11, 1918 – December 2, 1959), also called Jack O'Hara and "The Enforcer," was a criminal and freelance contract killer and bookie, who worked for the Los Angeles crime family, although he also was associated with Benjamin "Bugsy" Siegel, the Shannon brothers (Joe, Izzy, Moe, and Max) and Mickey Cohen during the 1940s and 1950s.

Whalen's father, Fred—who was also a career criminal—had enrolled Jack in military school to give his son a better life.

Jack played polo in school and piloted bombers in World War II.

"[3] On December 2, 1959, Whalen was shot "square between the eyes" and killed at Rondelli's restaurant in Sherman Oaks in the presence of Mickey Cohen and three other of his associates.

Confidential; as part of the power struggle for Mickey Cohen's criminal empire against gangster Morris "Mo" Jahelka.