Historically in Lancashire, and with little early history to speak of, Oldham rose to prominence in the 19th century as an international centre of textile manufacture. It was a boomtown of the Industrial Revolution, and among the first ever industrialised towns, rapidly becoming "one of the most important centres of cotton and textile industries in England". At its zenith, it was the most productive cotton spinning mill town in the world, producing more cotton than France and Germany combined. Oldham's textile industry fell into decline in the mid-20th century; the town's last mill closed in 1998.
The demise of textile processing in Oldham depressed the local economy. Today Oldham is a predominantly residential town, and the improvement of the town centre is the focus of a project for transforming Oldham into a centre for further education and the performing arts. It is, however, still distinguished architecturally by the surviving cotton mills and other buildings associated with that industry. The town has a population of 103,544 and an area of around 26 square miles (67 km2).
The toponymy of Oldham seems to imply "old village or place" from Eald (Saxon) signifying oldness or antiquity, and Ham (Saxon) a house, farm or hamlet. Oldham is however known to be a derivative of Aldehulme, undoubtedly an Old Norse name. It is believed to be derived from the Old English ald combined with the Old Norse holmi or holmr, meaning "promontory or outcrop", possibly describing the town's hilltop position. It has alternatively been suggested that it may mean "holm or hulme of a farmer named Alda". The name is understood to date from 865, during the period of the Danelaw.
The earliest known evidence of a human presence in what is now Oldham is attested by the discovery of Neolithic flint arrow-heads and workings found at Werneth and Besom Hill, implying habitation 7–10,000 years ago. Evidence of later Roman and Celtic activity is confirmed by an ancient Roman road and Bronze Age archaeological relics found at various sites within the town. Placenames of Celtic origin are still to be found in Oldham: Werneth derives from a Celtic personal name identical to the Gaulish vernetum, "alder swamp", and Glodwick may be related to the modern Welsh clawdd, meaning "dyke" or "ditch". Nearby Chadderton is also pre-Anglo-Saxon in origin, from the Old Welsh cadeir, itself deriving from the Latin cathedra meaning "chair". Although Anglo-Saxons occupied territory around the area centuries earlier, Oldham as a permanent, named place of dwelling is believed to date from 865, when Danish invaders established a settlement called Aldehulme.
From its founding in the 9th century until the Industrial Revolution, Oldham is believed to have been little more than a scattering of small and insignificant settlements spread across the moorland and dirt tracks that linked Manchester to York. Although not mentioned in the Domesday Book, Oldham does appear in legal documents from the Middle Ages, invariably recorded as territory under the control of minor ruling families and barons. In the 13th century, Oldham was documented as a manor held from the Crown by a family surnamed Oldham, whose seat was at Werneth Hall.