O'Kane has appeared in multiple plays written by Owen McCafferty, including Closing Time at the National Theatre in 2002, Scenes from the Big Picture (playing a character who works in an abbatoir rooted in the playwright's own experience) at the National Theatre Cottesloe in 2003, and Shoot the Crow at the Royal Exchange Studio, Manchester in 2003. O'Kane and McCafferty grew up in the same Belfast village and attended the same youth club, though McCafferty was four years older. They both played Gaelic football for the local club. The two did not collaborate as theatrical professionals until McCafferty began writing Closing Time while on secondment at the National Theatre Studio.

He won a Nesta Fellowship in 2005 and has released a book "Actor's Voices" a series of conversations with modern actors (Oberon Press 2012).

He has been part of the companies of the Royal National Theatre, where he appeared in The Playboy of the Western World and Peer Gynt, and the Royal Shakespeare Company, where he played the title role in Macbeth. At the Royal Exchange Theatre, Manchester, he gave an "eye-catching", "triumphant" performance in the title role of Christopher Marlowe's Doctor Faustus in 2010.

In October 2011 O'Kane appeared to great acclaim in the lead role of "16 Possible Glimpses" at the Abbey Theatre in Dublin https://web.archive.org/web/20140625175228/http://www.abbeytheatre.ie/whats_on/event/16_possible_glimpses/

In May 2012, he appeared as J. Bruce Ismay in Owen McCafferty's Titanic (Scenes from the British Wreck Commissioner’s Inquiry, 1912) at the newly opened MAC art's venue in Belfast and will be performing in Owen McCafferty's play "Quietly" in the Abbey Theatre in October 2012.

In 2013/14 he appeared in Quietly in Edinburgh and moved on to Ireland,Germany and London. This was a 3 people theatre production and he played one of the two people arguing over an incident that had happened years before. He was playing the character Jimmy.