Onslow Gordon Douglas was born at Westbury, Tasmania, on 4 Aug 1861 and was christened at the Westbury Methodist Church on 5 August 1862. He was the twelfth child of Roddam Hulke Douglas (1817-1902) and his wife Mary Selina Langslow (1822-1895). Onslow was educated at Horton College, Ross. In 1880 he was articled to the firm of solicitors Douglas and Collins. In 1885 he was admitted to the Bar and in 1890 became a partner in his uncle's firm.
In his younger days Onslow played football and cricket. He was a member of numerous groups including the Northern Athletic Association, Northern Cricket Association and Horticultural Society. Onslow married Kate Frith on 11 Mar 1885 at St George's Church, Battery Point. They had three children, all born in Launceston: Roderick in 1885, Greta in 1891 and Colin in 1894. Onslow died on 17 Sep 1943 in Melbourne, Victoria.