When Frederick William Sanderson became headmaster in 1892, Oundle was a small country boarding school of around a hundred boys. By the time he died in 1922, having just finished giving a lecture at University College London to an audience that included H. G. Wells, it was one of the most influential schools in England. Sanderson believed boys should learn what they wanted to learn, and he believed engineering and the experimental sciences were as worthy of a gentleman's education as Latin verse composition. He filled the school with workshops, lathes, forges and laboratories at a time when most English public schools regarded such things as suspiciously commercial. He let pupils tinker with petrol engines. He talked to them as people. Wells wrote a biography of him after his sudden death that reads as a manifesto for what English education could have been if more headmasters had been like Sanderson. The Patrick Engineering Centre, the SciTec complex and the school's continuing reputation in the sciences all descend from those decades. The pupils still build cars from raw stock in the workshops he founded.

The Chapel of Saint Anthony was consecrated in 1923 as a memorial to the two hundred and twenty-one Old Oundelians killed in the First World War. The architect was Arthur Conran Blomfield; the roll-of-honour tablets in the ambulatory listing every name were designed by Bertram Clough Williams-Ellis, an Old Oundelian himself who later designed Portmeirion. In 1956, to mark Oundle's four hundredth anniversary, the Grocers' Company commissioned John Piper to design three new windows for the sanctuary. Piper had never worked in stained glass before. He turned to Patrick Reyntiens to interpret his modernist designs, and the partnership begun for Oundle would go on to produce around sixty stained glass commissions together, including the great windows at Coventry Cathedral and Liverpool Metropolitan Cathedral. The chapel is now one of the most important small spaces of twentieth-century English religious art. Elsewhere on the school's calendar of unexpected events, the experimental industrial group Throbbing Gristle played in 1980 in front of a bewildered audience of teenage boys, a performance that has since taken on something of an apocryphal glow.

The Old Oundelians club was founded in 1883. Among the names on its rolls are Sir Maxwell Hutchinson, past president of the Royal Institute of British Architects; the evolutionary biologist Richard Dawkins; Bruce Dickinson, the singer who fronts Iron Maiden and flies airliners; the England rugby twins Tom and Ben Curry; the architect Christopher Alexander, whose pattern-language ideas influenced both architecture and the early development of software design; and Caroline Criado Perez, whose campaigning brought Jane Austen onto the ten-pound note. Three Old Oundelians were awarded the Victoria Cross during the First World War: Alan Jerrard, Cecil Knox and Charles Vickers. Vickers, who survived to old age, went on to become a foundational figure in systems theory and a senior civil servant. The school admitted girls for the first time in 1990 and reabsorbed its day-school sibling Laxton in 2000.

Oundle is unusual in that the school is not on a separate campus walled off from the town: its buildings are scattered through the streets, the Cloisters in the centre, the Stahl Theatre in a converted church on West Street, the SciTec complex and Patrick Engineering Centre on the south side, boarding houses tucked along the main thoroughfares. The school's full-bore outdoor rifle range, five hundred yards long, sits outside the hamlet of Elmington on the Ashton estate and is one of the few schools-owned ranges of its size in the country. The Combined Cadet Force is the largest of any school in Britain. Every summer since 1982, sixth formers have run a residential holiday for children with learning disabilities, now a registered charity in its own right. Around eleven hundred pupils live and study here, the third-largest boarding school in England after Eton and Millfield, and most of the town's day-to-day rhythm is shaped by where they are in the school year.