John O'Callaghan may refer to:John O'Callaghan (1850–1899), American soldier and Medal of Honor recipient John O'Callaghan (politician), secretary of the United Irish League John O'Callaghan (musician), Irish musician and DJ Liam McCarthy and John D. O'Callaghan, BT Young Scientists of the Year and EU Young Scientists of the Year, 2009 John Cornelius O'Callaghan (1805–1883), Irish historian and writer John Joseph O'Callaghan (1838–1905), Irish architect John O'Callaghan (singer), vocalist and guitarist of The Maine John O'Callaghan, American football player in the 1987 Seattle Seahawks season