William P. O'Brien (June 15, 1891 – December 17, 1960) was an American law enforcement officer who served as New York City Police Commissioner from 1949 to 1950.

During World War I he served in the Military Police Corps of the United States Army and was discharged with the rank of lieutenant.

However, one day before O'Brien was to take office, commissioner Arthur W. Wallander announced his resignation, meaning that O'Brien would serve as acting commissioner while Wallander spent his final month in office on vacation then succeeded him on March 1.

On September 15, 1950, MacDonald's men raided a $20 million betting syndicate and arrested bookmaker Harry Gross.

After O’Brien was presented with wiretap evidence of Gross paying off police officers he announced the department would begin its own investigation into the matter.