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.
