 O'Brien made his name playing opposite Ethel Barrymore, in a revival of Sir Arthur Wing Pinero's play "Trelawny of the 'Wells'", which opened on New Year's Day, 1911, at the Empire Theatre.O'Brien's other Broadway credits included "The Country Cousin" (1917), "Her Husband's Wife" (1917), "The Angel in the House" (1915), "The Bargain" (1915), "A Celebrated Case" (1915), "The Money Makers" (1914), "A Woman Killed with Kindness / Granny Maumee" (1914), "Kitty Mackay" (1914), "Tainted Philanthropy" (1912), "The Case of Becky" (1912), and "The Million" (1911).O'Brien's first film, Essanay Film's "The Lieutenant Governor", in which he had the starring role, played in Boulder's Curran Theater in February 1915, giving his family its first opportunity to see him act.