Oxford bags were a loose-fitting baggy form of trousers favoured by members of the University of Oxford, especially undergraduates, in England during the early 20th century from the 1920s to around the 1950s. The style had a more general influence outside the University, including in America, but has been somewhat out of fashion since then.
The style originated from a ban in 1924 on the wearing of knickers by Oxford (and Cambridge) undergraduates at lectures. The bagginess allowed knickers to be hidden underneath easily. The style was invented by Harold Acton of Christ Church.
The style made a comeback in 1970s Britain, often worn with platform shoes. A popular 1970s Scottish boy band, The Bay City Rollers wore a variant of oxford bags with tartan trimmings that fell short of ankle-length.