O'Reilly was born in Swansea in South Wales and raised in Dublin, Ireland until he moved with his family to Vancouver, British Columbia, Canada at the age of nine.

[citation needed] After high school he became a Toronto-based arts correspondent for The Globe and Mail and then spent three years writing pop culture and entertainment pieces for the National Post.

[citation needed] He turned to photography in 2005 and from 2005 until 2012 was the Reuters Chief Photographer for West and Central Africa, based in Dakar, Senegal.

[citation needed] In 2012 he took a sabbatical year off to study psychology as a Nieman Fellow at Harvard with a focus on conflict-induced trauma.

[citation needed] O'Reilly is one of several journalists included in Under Fire: The Psychological Cost of Covering War, a documentary which won a 2013 Peabody Award.