John Oswald Sanders was born in Invercargill on 17 October 1902, the son of Alfred Sanders, an accountant, and his wife, Margaret Menzies Miller. Oswald was educated at Southland Boys' High School, and at the age of 15 was employed as a clerk by the law firm of Frederick Hall-Jones. While there he studied extramurally towards a law degree.
The family belonged to the Brethren assembly in Invercargill and were also involved in the South Island committee of the China Inland Mission. Oswald experienced a childhood conversion in 1911, but the turning point of his life came at a conference for spiritual awakening in Pounawea in 1921 when he dedicated himself to missionary service. He declined a partnership in Hall-Jones's firm, deciding instead to prepare for missionary service in South America at the New Zealand Bible Training Institute (NZBTI) in Auckland.
His study ended abruptly in October 1924 when his father became ill and he was called home to support his parents. The three soon shifted to Dunedin, where Oswald was employed by John Wilkinson, a prominent lawyer and evangelical lay preacher. In 1926 he was invited to serve as the field representative of the NZBTI, and later that year he became secretary and treasurer, moving his parents to Auckland. There, on 19 December 1931, he married Edith Mary Dobson at the Baptist Tabernacle.
Sanders was increasingly drawn into every aspect of the teaching and administration of the rapidly growing Bible institute, succeeding C. J. Rolls as superintendent in 1931. After the death of the principal and founder, Joseph Kemp, in 1933, he took effective charge. He never spared himself in the task of developing the institute. He also founded a Christian bookshop, conducted weekly mission services, and supported and spoke regularly at the annual Easter convention at Ngaruawahia. Sanders encouraged a wide variety of missionary organisations, and was instrumental in the formation of the United Maori Mission in 1936.
His reputation as a forthright, unpretentious and compelling preacher led to an invitation to become the Australasian representative of the China Inland Mission from March 1946. He was based in Melbourne and in 1954 was appointed general director of the mission. This large organisation was in crisis after its expulsion from China, and Sanders would lead its reorganisation into the Overseas Missionary Fellowship. Working from Singapore, he became well known in evangelical circles throughout the world for his efficiency, his skill in reasoning and planning, and his ability to listen.
In the 1920s Oswald Sanders had written magazine articles for the NZBTI magazine, the Reaper. In 1937 he began writing books, particularly on the spiritual life and on qualities of Christian leadership. All told he published over 30 books: more than two million copies were printed, including translations into some 23 languages.
Edith Sanders died in Melbourne in 1966. At the Bible Training Institute in Henderson on 21 September 1968, Sanders married Mary Christian Miller, a teacher, who was a widow and the daughter of Joseph Kemp. After his retirement to Auckland in 1969, Sanders received a flood of invitations from institutions all over the world, and over the next 23 years he was constantly travelling and fulfilling speaking engagements. Mary died in 1972 and Oswald spent 1973–74 as principal of the Christian Leaders' Training College in Banz, New Guinea. He was made an OBE in 1980 and received an honorary doctorate of theology in 1992 at the 70th anniversary celebrations of his old institute (by then called the Bible College of New Zealand). He died, aged 90, in Auckland on 24 October that year, survived by the son of his first marriage. His funeral service was held at the Mount Albert Baptist Church.
Called J.O. or 'Ossie' by his friends, Sanders was a man of commonsense and wisdom, not easily swayed by emotion, and a natural if austere leader. His favourite subject as a preacher was the life of Caleb, the Old Testament character who to the end of his life sought opportunities to conquer mountains. Sanders helped to develop in New Zealand a strong, positive evangelicalism with a consciously international outlook.