A member of the Royal Shakespeare Company (RSC) 2009-2011 ensemble, his roles included Mercutio in Romeo and Juliet, Orlando in As You Like It, and Launcelot in Morte D'Arthur. His performances during the RSC's six-week residency at Park Avenue Armory in New York were hailed as"forceful" and"irresistible." At the 2012 World Shakespeare Festival in Stratford-upon-Avon, O'Neill played the title role in Roxana Silbert's production of Richard III at the Swan Theatre.
In 2012 he won praise for his performance in Lucy Prebble's play The Effect at the Royal National Theatre headlining alongside Billie Piper, whom he later appeared alongside in the 2013 fiftieth anniversary episode of Doctor Who:"The Day of the Doctor". He also appeared in"The Mortal Remains," the final vignette in the Coen brothers's film The Ballad of Buster Scruggs (2018).