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 Miami Police Chief. He resigned as chief on April 28, 2000, in the aftermath of the Elián González custody battle raid. The resignation has been studied as a case in criminal justice and political ethics. He was highly critical of the Miami mayor's handling of the situation, leading to his forced resignation.