The settlement was first mentioned about 780 whenCharlemagne in course of theSaxon Wars crossed theOker river and had a church dedicated toSaint Stephen erected at a place then calledSalingenstede. This church became a centre of the Christian mission among the paganSaxons, overseen byHildegrim of Châlons, and the origin of the laterBishopric of Halberstadt. On 1 April 974, EmperorOtto II issued a deed grantingmarket andmint rights toSeligenstadt, this is commonly regarded as the date of the town's foundation.
The name Osterwieck was first mentioned in a 1073 letter by ArchbishopLiemar of Bremen to BishopBurchard II of Halberstadt. Destroyed by a blaze in 1511, the town was rebuilt and today features an ensemble of about 400Fachwerk buildings. The nave of the St. Stephen Church erected in the 16th century is one of the oldestProtestant church buildings, while itsRomanesque twin steeples date back to 1100.