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 O'Hennessy made his first fan based personal appearance at the Wrexham Comic Con event in November 2015.
