He is best known for his roles in Hollyoaks Later, Da Vinci's Demons and The Musketeers. In 2015, he replaced Edward Dogliani as Lord of Bones in Season 5 of the HBO series Game of Thrones before moving on to play the Role of Sir Locke in Kurt Sutter's new international television series The Bastard Executioner. At the age of 18 he left wales and moved to London to chase his career.
He was extremely lucky to be accepted in to the National Youth Theatre where he was tutored by actor Hakeem Kae-Kazim and performed in Maggie May, The Tempest and Macbeth in London. O'Hennessy picked up a career in the theatre and gained his first appearance at Chichester Festival Theatre in a play called The Visit in which he acted opposite Hollywood actress Lauren Bacall which was directed by Terry Hands. With such a big production came a lot of attention for Ross and so he was asked the join The Royal Shakespeare Company where he performed in As You Like It directed by Steven Pimlott, Three Hours After Marriage directed by Richard Cottrell and Troilus and Cressida directed by Ian Judge. and film appearances O'Hennessy very quickly moved into the world of television and film with his first television appearance being as Cpl Dando on Soldier Soldier in 1995 to 1997.
He continued to work as a typical television jobbing actor until he got his big break in 2011 playing the role of Bruce Maverick in Hollyoaks Later. He was asked to play the role of Commander Quattrone in Da Vinci's Demons. having played this powerful role in such a big budget production allowed O'Hennessy to move into performances like Rattle Shirt - The Lord of Bones in Game of Thrones. Ross has completed work in 2015 on the BBC Musketeers and has begun working on Kurt Sutter's new show all about the medieval times.
O'Hennessy plays Sir Locke in this new FX production called The Bastard Executioner. Ross O'Hennessy made his first fan based personal appearance at the Wrexham Comic Con event in November 2015.