Osterode is a district in Lower Saxony, Germany. It is bounded by (from the southwest and clockwise) the districts of Göttingen, Northeim and Goslar, and by the state of Thuringia (districts of Nordhausen and Eichsfeld). This part of the Harz mountains was ruled by the Welfen dynasty from the 12th century on. Osterode was the centre of the Principality of Brunswick-Grubenhagen, one of many small states within Brunswick-Lüneburg. Later this principality became part of Hanover, which in turn fell to the Kingdom of Prussia in 1866. In 1885 the Prussian administration established districts, among them Osterode.