He joined the Irish Volunteers in 1913 and by 1916 he was a captain in command of A Company, 3rd Battalion, Dublin Brigade under Éamon de Valera.
When the vice-commandant failed to show for the 1916 Easter Rising de Valera made O'Connor his second in command.
Returning to Dublin O'Connor joined "The Squad" under Michael Collins and killed Captain John FitzGerald of the "Cairo Gang" on Bloody Sunday in 1920.
In the Battle of Dublin during the Irish Civil War he held the Fianna HQ in York Street near St Stephen's Green.
[4] Despite joining Fine Gael he presented de Valera with a scroll of those killed in the Easter Rising in a 20th anniversary remembrance.