William, Billy or Bill O'Neill may refer to:

• Bill O'Neill (Ohio politician), former member of the Ohio House of Representatives
• Bill O'Neill (New Mexico politician), member of the New Mexico Senate
• C. William O'Neill (1916–1978), Governor of Ohio
• William O'Neill (Ohio judge) (born 1947), appellate judge and 2008 candidate for U.S. Representative from Ohio's 14th congressional district
• William A. O'Neill (1930–2007), Governor of Connecticut, 1980–1991
• William P. O'Neill (1874–1955), Lieutenant Governor of Indiana

• Bill O'Neill (American football) (1910–2000), American football player
• Bill O'Neill (baseball) (1880–1920), American baseball player
• Bill O'Neill (bowler) (born 1981), American professional ten-pin bowler
• Bill O'Neill (bowls) (1909–?), New Zealand lawn bowler
• Billy O'Neill (rugby) (1878–1955), Welsh rugby player
• Billy O'Neill (dual player) (1929–2015), Irish former Gaelic football and hurler
• Billy O'Neill (footballer) (born 1919), Irish footballer
• Willie O'Neill (footballer, born 1940) (1940–2011), Scottish footballer (Celtic FC)
• Willie O'Neill (Wexford hurler) (born 1945), Irish hurler
• Willie O'Neill (Cork hurler) (1876–1963), Irish hurler and Gaelic footballer
• Willie O'Neill (Irish footballer), Irish international footballer

• Bill O'Neill (media) (born 1936), News Corporation executive
• Buckey O'Neill (1860–1898), Old West sheriff and U.S. Army officer killed at the Battle of San Juan Hill
• William F. O'Neil, founder of General Tire and Rubber Company
• William O'Neill, 1st Baron O'Neill (1813–1883), Anglo-Irish hereditary peer, clergyman and musical composer
• William O'Neill (Medal of Honor) (1848–?), American soldier in the Indian Wars

• William O'Neil (disambiguation)
• Bill O'Neal, American historian; see Bibliography of Wyoming history
• Billy O'Neil, fictional footballer in Dream Team
• William O'Neal (disambiguation)