William O'Brien (March 10, 1944 – March 3, 2016) was an American police officer who served as the 21st police chief of Miami, Florida, from 1998 until 2000. O'Brien served as a Miami Police Department police officer for more than 25 years, including 18 years as a member of the city's SWAT team and two years as chief.^[1][2][3] He resigned as chief on April 28, 2000, in the aftermath of the Elián González custody battle raid.^{[3][4][5][6][7][8][9]} The resignation has been studied as a case in criminal justice and political ethics.^[10][11][12] He was highly critical of the Miami mayor's handling of the situation, leading to his forced resignation.^[13][14][15]

Bill O'Brien, who was raised in La Grange, Illinois, first moved to Miami to study political science at the University of Miami.^[3] He then joined the United States Air Force for five years, during which he piloted C-130 Hercules in combat during the Vietnam War.^[3] O'Brien sought to become a professional pilot after the war, but could not find a job in the field during the oil embargo in 1973.^[3] Instead, O'Brien became a police officer, despite his dislike of guns.^[3]

O'Brien died from throat cancer at his home in Tavernier, Florida, on March 3, 2016, at the age of 71.^[3]