Oldenburg (Ol-den-boorg), a grand-duchy of northern Germany, consisting of three distinct territories - Oldenburg Proper, the principality of Lubeck, and Birkenfeld. Total area, 2508 sq. m. (less than Devonshire); pop. (1900) 398,499. Oldenburg Proper, which comprises 7/8ths of this area, is bounded by the German Ocean and Hanover. The principal rivers are the Weser, the Jahde, and various tributaries of the Ems. The country is flat, part of the great sandy plain of northern Germany, and mainly moors, heaths, marsh or fens, and sandy tracts. The principality of Lubeck, consisting of the secularised bishopric of the same name, does not contain the city (north of which it lies), and is surrounded by Holstein. Its area is 209 sq. m. The principality of Birkenfeld (q.v.) lies among the Hundsruck Mountains, in the very south of Rhenish Prussia; its area is 192 sq. m. Oldenburg became an independent state in 1180. The family that then established its power has continued to rule to the present day, giving, moreover, new dynasties to Denmark, Russia, and Sweden. Danish from 1667 to 1773, Oldenburg acquired the Lubeck territories in 1803, and Birkenfeld at the Congress of Vienna, when it became a grand-duchy.
The capital, Oldenburg, is pleasantly situated on the Hunte, 30 miles WNW. of Bremen by rail. It has the grand-ducal palace, with fine gardens and art collections, a public library of 100,000 volumes, a picture-gallery, museum, etc. Oldenburg is the seat of an active river-trade, and is noted for its great cattle and horse fairs. Pop. 28,000.