Oliver Duff was born at Waitahuna Gully, Otago, on 28 May 1883, the 10th of 13 children of English-born William Duff and his Scottish wife, Margaret Shepherd. His parents had kept a general store at Waitahuna since the goldrush of the 1860s, but by the time of Oliver's birth, trade was receding to the larger town of Lawrence. While he was still a small child the family moved to a large farm, which they called Gowan Braes, on tussock hills at Edievale, and the farming routines became embedded in Oliver's consciousness as the ideal life. He received his secondary education at Lawrence District High School and Otago Boys' High School. Scarcely out of school and only 18, he volunteered for the South African war, sailing from Lyttelton with the Eighth Contingent on 8 February 1902.
Although Duff was in South Africa only during the last stages of the war, his experiences made a profound impression on him. On his return to New Zealand he sought to offer humanitarian service through the Presbyterian ministry. He was awarded a scholarship in 1903 by the Synod of Otago and Southland to study for a university degree, a prerequisite for ministerial training, and spent two years studying for a BA. By then he had rejected much of the church's dogma, partly through his reading of free-thinking but still strongly ethical writers such as Emerson and Thoreau. Rolling a swag and pocketing the Bible and Burns's poems, he took to the road, doing odd jobs for farmers while he thought through his position. In 1905 he withdrew from training for the ministry. Instead, he taught in technical schools in Nelson and Invercargill before completing his BA in 1914. On 12 October 1908 at Dunedin he married Jessie Barclay, daughter of a hotel-keeper in Lawrence. They were to have three sons and a daughter.
In 1916 Duff made a decisive change of occupation by joining the staff of the Sun, an enterprising daily newspaper in Christchurch. In 1920 he became editor of the Timaru Herald, where he earned a reputation for the clarity and wit of his editorial writing, and in 1923 he returned to Christchurch as editorial assistant on the Press.
M. C. Keane, editor from 1919, had revived the Press's literary tradition, and Duff was to continue this when he was appointed editor two days after Keane's early death in 1929. Together they encouraged and printed local writers, who included Ngaio Marsh, M. H. Holcroft and D'Arcy Cresswell. Even as editor the sturdy, heavily eyebrowed Duff cultivated the appearance of a countryman, walking to the office from his Riccarton home in a tweed suit, with stick and dog. His tenure was brief, however. In 1932 he resigned, typically on a point of principle, when the paper's proprietors tried to interfere in his coverage of the Christchurch tramway strike, which they thought too sympathetic to the unions.
Several difficult years followed. He took over the Record, a weekly paper in Rangiora, re-establishing it as the North Canterbury Gazette. It was aimed at farmers while again having a literary flavour. From 1936 he attempted with little success to make a living from a small property in the Lansdowne valley, running into the Banks Peninsula hills south of Christchurch. In 1937 he and Jess were divorced; as Jess Whitworth, she later published a prize-winning autobiographical novel, Otago interval (1950).
Duff's fortunes revived when, in 1937, he wrote to J. W. Heenan, under-secretary of internal affairs, asking for work with the forthcoming centennial publications. He was appointed editor in 1938. By the end of that year, as head of a team similarly recruited from the ranks of the penurious, he had established the programme, commissioned writers, and contracted printers for one of the country's most notable publishing enterprises. His own contribution to it was New Zealand now (1941), a personal view full of paradox and epigram, showing the maturity of a style that had been formed in an age of journalist-stylists such as G. K. Chesterton.
Early in 1939 Duff was appointed founding editor of the New Zealand Listener. The director of the National Broadcasting Service, James Shelley, intended the new journal to be his own mouthpiece. The editor was instructed that everything published was to relate to broadcasting and that he was to show the contents of each issue to the director before printing. But Shelley met his match in the strong-minded Duff. Stressing the impracticality of submitting all decisions to the director, Duff gradually asserted his independence, and during the difficult war years turned the Listener into New Zealand's closest approach after the demise of Tomorrow in 1940 to a weekly review of world events and cultural affairs – although political controversy was excluded. He published many young writers, of whom Frank Sargeson was the most talented. With an assured circulation because of its monopoly of radio programme listings, the Listener otherwise paid minimal attention to broadcasting affairs. To the protests of broadcasters Duff turned a deaf ear, and set out to form, rather than be formed by, his readers' tastes. The result was a lively and stimulating journal. His own contribution was the weekly editorial, which in a mere 300 words (his successor required twice as many) cut to the bone of many passing issues.
Duff married Ngaire Asquith Shankland at Wellington on 29 January 1946, and after his retirement in 1949 they lived on his Lansdowne valley smallholding, Spylaw. From there, under the pseudonym 'Sundowner', he wrote a weekly column for the Listener, 'Shepherd's Calendar', from which a selection under the same title was published in 1962. He followed the farming seasons, ruminated on the moral ambiguity of raising animals for slaughter, and revealed consciously and unconsciously how strongly both his Bible-centred upbringing and the philosophies of Emerson and Thoreau remained with him.
Duff died at his home on 2 March 1967, survived by Ngaire and the children of his first marriage. One son, Roger, became director of the Canterbury Museum and a pioneering authority on the moa-hunter period of Maori culture. His daughter, Alison, was a noted sculptor, and another son, Gowan, a scientist, was father of the novelist Alan Duff.