O'Reilly grew up in Victoria and in 1894 joined the rush to the Coolgardie goldfield in Western Australia while still a teenager. His two years on the goldfield delivered over 2,000 pounds into his bank account. After falling in love with a Perth barmaid who defrauded him of much of his money, he went back to the Western Australian mine fields. After accumulating another 1,800 pounds he set off for France and England in 1900 with his mates. He soon returned and spent further years prospecting around Geraldton and the north of Western Australia before moving on to Darwin and Central Australia. In 1912 he met the Reverend John Flynn, later known as 'Flynn of the Inland'. O'Reilly enlisted in 1914 but was discharged as medically unfit in January 1915. He went back to Marranboy in the Northern Territory where tin mining proved profitable. O'Reilly later mined for opals at Coober Pedy in South Australia and sold his opals in Adelaide for 4,000 pounds. The next few years were spent around Broken Hill before moving to Tasmania to mine osmiridium. Age forced O'Reilly to retire from mining. In 1941 he enlisted in the Militia and was discharged in 1943.
O'Reilly's service record in the World War II Nominal Roll gives his date of birth as 29 August 1891. However, we know from his autobiography, Bowyangs and Boomerangs (1944), that he was an adolescent when he headed for the West Australian goldfields in 1894. It is probable he altered his age to enlist in the Militia in 1941. There seems no reason to doubt his assertion in the Nominal Roll that he was born in Ireland or that his next of kin was Louisa O'Reilly.