Oldenburg, a grand-duchy of northern Germany, consisting of three distinct and widely separated territories—viz. Oldenburg Proper, the principality of Lübeck, and the principality of Birkenfeld. Total area, 2508 sq. m. (less than Devonshire); pop. (1890) 354,968. Oldenburg Proper, which comprises ths of this area, is bounded by the German Ocean and Hanover. The principal rivers are the Weser, the Jahde, and the Haase, Vehne, and other tributaries of the Ems. The country is flat, belonging to the great sandy plain of northern Germany, and consists for the most part of moors, heaths, marsh or fens, and sandy tracts. The occupations are mainly agricultural, with some iron-working, fisheries, and shipping; there is also a little wool-spinning and linen-weaving. The principality of Lübeck, consisting of the secularised territories of the former bishopric of the same name, does not contain the city (north of which it lies), and is surrounded by the duchy of Holstein. Its area is about 200 sq. m. The principality of Birkenfeld (q.v.) lies among the Hundsriück Mountains, in the very south of Rhenish
Prussia, by which it is surrounded; its area is 192 square miles.
Oldenburg is a constitutional ducal monarchy. The constitution, which is based upon that of 1849, revised in 1852, is common to the three provinces, which are represented in one joint chamber. Each principality has, however, its special provincial council. The grand-duke has a civil list of £12,570, besides private revenues.
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 the kingdom of Denmark, the empire of Russia, and the kingdom of Sweden. On the death, in 1667, of Count Anthony Gunther, the wisest and best of the Oldenburg rulers, his dominions fell to the Danish reigning family, and continued for a century to be ruled by viceroys nominated by the kings of Denmark. This union was, however, severed in 1773, when by a family compact Christian VII. made over his Oldenburg territories to the Grand-duke Paul of Russia, who represented the Holstein-Gottorp branch of the family. Paul having given up Oldenburg to his cousin, Frederick-Augustus, of the younger line of the House of Oldenburg, the emperor raised the united Oldenburg territories to the rank of a duchy. For a time the duchy was a member of Napoleon's Rhenish Confederation. The Lübeck territories were added in 1803; Birkenfeld at the Congress of Vienna, when Oldenburg became a grand-duchy. The grand-duchy concluded in 1866 a treaty with Prussia, by which the grand-duke renounced his claims to the Holstein succession. See SLESWICK-HOLSTEIN.
The capital, OLDENBURG, is pleasantly situated on the banks of the Hunte, 30 miles WNW. of Bremen by rail. It is the focus of the literary, scientific, and commercial activity of the duchy, and has a public library of 100,000 volumes, a picture-gallery, museum, &c. The grand-ducal palace is worthy of note for its fine gardens, its valuable pictures and other art collections, and its library. The principal church, St Lambert's (1270), contains the burying-vaults of the reigning family. Oldenburg is the seat of an active river-trade, and is noted for its great cattle and horse fairs. Pop. (1875) 15,701; (1890) 21,646. See Runde's Oldenburgische Chronik (3d ed. 1863).