Born to a musician mother, Denis had the skills of an artist instilled in him at a tender age which is quite evident from his pursuits. Even when was a teenager Denis contributed actively to his school’s choir. He landed his very first audition in 1974 where he gained a part in the chorus of the play Show Boat. After that his life has been nothing short of a vibrant palate of varied roles.
After having studied theater at the School of Communication at the Northwestern University, this Irish actor plunged into the world of acting, his work ranging from plays to TV shows to movies. O'Hare received the Tony Award for the Best Performance for Richard Greenberg’s play “Take Me Out” wherein his character has prolonged monologs which were heavily lauded and reflected as to why he deserved the award. He received the Outstanding Featured Actor Award for the Drama Desk Award because of his Broadway role in the musical Sweet Charity.
He received the Musical Tony Award nomination for the Best Performance by a Featured Actor for the Broadway production of Stephen Sondheim’s Assassins. His acknowledged works include 21 Grams, Cabaret, Brothers & Sisters and Saint Maybe. Denis has done guest roles in the show Law & Order as well its contemporary shows like Law & Order: Criminal Intent and Law & Order: Special Victims Unit.. His film credits include Changeling, Charlie Wilson's War, Edge of Darkness, Milk, Half Nelson, A Mighty Heart, Michael Clayton, Derailed, Garden State, 21 Grams, and The Anniversary Party. , and his TV credits include An Englishman in
New York, American Horror Story, Bored to Death, True Blood,
The Eagle, The Good Wife, and Murder House to name a few.
The fact that he’s mostly playing vile characters hasn’t gone unnoticed by Denis, who quipped, “I probably killed more than 587 people on True Blood,” referring to his character as a vampire Russell Edgington, on HBO’s supernatural sudsier. He came out gay while in High School and is now happily married to Hugo Redwood since July 28, 2011, and the couple have also adopted a son, Declan.