The settlement was first mentioned about 780 when Charlemagne in course of the Saxon Wars crossed the Oker river and had a church dedicated to Saint Stephen erected at a place then called Salingenstede.

This church became a centre of the Christian mission among the pagan Saxons, overseen by Hildegrim of Châlons, and the origin of the later Bishopric of Halberstadt.

On 1 April 974, Emperor Otto II issued a deed granting market and mint rights to Seligenstadt, this is commonly regarded as the date of the town's foundation.

The name Osterwieck was first mentioned in a 1073 letter by Archbishop Liemar of Bremen to Bishop Burchard II of Halberstadt.

On 1 January 2010 the former municipalities Aue-Fallstein, Berßel, Bühne, Lüttgenrode, Rhoden, Schauen, and Wülperode were merged into Osterwieck.