Major-General Denis O’Reilly grew up in Moose Jaw, SK and enrolled in the Canadian Armed Forces in 1991. He received his RCAF pilot wings in 1995, and was posted to 408 Tactical Helicopter Squadron (Edmonton, AB) where he flew the CH135 Twin-Huey and later the CH146 Griffon helicopters.

Upon return to Canada in 2003, he served in various Flight Commander and operational positions in Gagetown, Halifax, and Kingston. Promoted to the rank of Colonel in 2015, he was appointed Director Air Force Training until 2016, when he returned to his home town of Moose Jaw, to assume command of 15 Wing - home of RCAF pilot training and The Snowbirds.

Major-General O’Reilly was posted in August 2020 to 2 Canadian Air Division as the Commander. Following two years leading individual training and education for the RCAF he was posted in August 2022 to the Office of the Chief of the Defence Staff as Chief of Staff and in June 2023 appointed as Commander, Canadian Defence Academy. He was promoted to his current rank in August 2023.

Major-General O’Reilly served on operational deployments to Haiti (1996), Kosovo (1999), and Kuwait (2014). He is a graduate of the Advanced Aerospace Operations Course, the Joint Command and Staff Program, and the United States Air Force Air War College. He holds a Bachelor of Philosophy degree, and a Masters of Defence Studies. He was appointed to the Order of Military Merit in November 2019.