Oldenburg (ôl´dənbŏŏrkh), former state, NW Germany. It is now included in the state of Lower Saxony. The city of Oldenburg was the capital. The former state consisted of three widely separated divisions. The largest of these, Oldenburg proper, now forms the district of Oldenburg, stretching S from the North Sea, W of the Weser River; the two other divisions, both very small, were Birkenfeld and the district (but not the city) of Lübeck. Oldenburg proper is a low-lying, fertile, and marshy land. The history of Oldenburg proper is mainly of dynastic significance. Originally a part of Saxony, the county of Oldenburg came into prominence in the 12th cent., when the counts became princes of the empire. In 1448, Count Christian became king of Denmark as Christian I, while his younger brother, Gerard, and his successors continued to rule Oldenburg. On the extinction (1667) of the German line, Oldenburg passed (1676) to Christian V of Denmark (direct descendant of Christian I). In 1773, Christian VII exchanged Oldenburg for ducal Holstein with Grand Duke (later Emperor) Paul I of Russia. Paul gave Oldenburg to his maternal great uncle, Frederick Augustus of Holstein-Gottorp, bishop of Lübeck, who assumed (1777) the ducal title. Peter I of Oldenburg, nephew and successor of Frederick Augustus, lost the duchy to Napoleon I but recovered Oldenburg and the bishopric of Lübeck in 1813 and subsequently acquired Birkenfeld and obtained the title grand duke. A member of the German Confederation from 1815, Oldenburg sided (1866) with Prussia in the Austro-Prussian War and joined (1871) the German Empire. The last grand duke abdicated in 1918, and Oldenburg joined the Weimar Republic.

Oldenburg, city (1994 pop. 147,701), Lower Saxony, NW Germany, on the Hunte River and the Küstenkanal (Coast Canal). It is a rail junction, transshipment point, agricultural market, and industrial center. Manufactures include ships, glass, and textiles. Oldenburg was first mentioned in 1108 and was chartered in 1345. It was the seat of the counts of Oldenburg until 1667, when it passed, with the county, to Denmark. From 1777 to 1918 it served as the residence of the dukes (later grand dukes) of Oldenburg. Noteworthy buildings include the former ducal palace (17th–18th cent.) and the Gothic Lambertikirche, a church built in the 13th cent. (rebuilt 18th–19th cent.).