[citation needed] He knocked out Dido Plum, the British middleweight champion, in six rounds, and George Crisp, the heavyweight titleholder, in eleven.

[3] O'Brien returned to Philadelphia in May 1902 and on December 20, 1905, won the world light heavyweight championship with a 13-round RTD over Bob Fitzsimmons in San Francisco, California, but abandoned the title without ever defending it.

He fought the fearsome middleweight champion Stanley Ketchel in a 10-round No Decision on March 26, 1909, in which O'Brien was saved by the bell at the end of the 10th round.

He fought heavyweight champion Jack Johnson in a six-round No Decision on May 19, but on June 9 he faced Ketchel again and was beaten in three rounds.

Hagan managed a gym on the seventh and top floors of the Rosemont building at 1658 Broadway, New York City, in the late 1920s/early 1930s.

[5] Philadelphia Jack O'Brien is a featured character in The Killings of Stanley Ketchel (2005), a novel by James Carlos Blake.