Born in Dublin, John left home at 17 to study Computer Science at the University of Ulster in Belfast. He started to act at the same time with Belfast Youth and Community Theatre - with a mandate to bring protestant and catholic actors together. The New Theatre school followed in Boston, Massachusetts. Theatre school graduation followed and he decided to pursue his craft in Toronto, Canada. He quickly became a member of Canadian Actors Equity and a working actor.
With an impressive resume he moved to New York City to star in the one man show Rum and Vodka written by Conor Mcpherson. This was a huge success. He went on to perform the show for NBC and HBO, selling the show out night after night. Other New York credits include The Comedians at La Mama etc. and The Hostage at the Irish Repertory Theatre.
After several Regional (including The Cleveland Playhouse) and other New York successes he moved to Los Angeles where he now resides with his adopted Ugandan son Odin (b. 2000) and dog Charlie.
He performs regularly with California's Artists Radio Theatre and LA Shorts at the Getty.
In 2008 has ended years of speculation in the hope of raising awareness of African adoption, and announced he was in a relationship since 2007 with Jaason Simmons, and that they plan to wed in Canada, where same-sex marriages are legally recognized.
Jaason Simmons (right) and John O' Callaghan, with their son Odin