His private diaries record that he personally celebrated his birthday on 2 July until 1947, but then apparently shifted the date by two days to emphasise his American origins. As a result, O'Malley's obituaries listed his date of birth as 4 July 1854. During his political career, O'Malley claimed to have been born in Quebec, at a location called "Stanford Farm" close to the U.S. border. He stated that no birth certificate existed, as registration of births was not yet standard in frontier regions. This account of his birth differed from those he related at other periods of his life. In an 1893 letter to the editor of the "Adelaide Advertiser", he proclaimed himself "a humble sovereign citizen of that supreme nation, the United States". Towards the end of his life he wrote "I am an American [...] that is the truth". O’Malley's place of birth had implications for his status as a member of parliament. If he was born in Canada, as he claimed, he was a British subject and faced no eligibility restrictions. If he was born in the United States, his election to federal parliament was in violation of section 44(i) of the constitution; similar provisions applied in South Australia and Tasmania, where he also ran for parliament. O’Malley's claim of Canadian birth was received with skepticism during his lifetime, and has generally been regarded as unreliable. Although he was unable to provide any proof beyond a sworn affidavit, political opponents were also unable to conclusively prove that he was born an American. On his 1910 marriage certificate, O’Malley listed his parents as Ellen (née King) and William O’Malley, and his father's occupation as rancher. His unusual given name was supposedly taken from his mother's family name, a common practice in the United States. His parents’ identities have not been corroborated, and other information that he gave on the certificate was of dubious accuracy. In 1913, O’Malley stated that his father was born in Ballymena, in the north of Ireland. The following year, he said that his parents were British subjects born in the United Kingdom (which included Ireland at the time). However, in old age he referred to his “American-born parents”. O'Malley claimed to have a brother and sister, and apparently stayed with his brother Walter in the small community of Kelly, Kansas, when he returned to the U.S. in 1917. O'Malley grew up in Kansas, possibly in Pawnee County.