OESEL, one of the largest islands in the Baltic, forming with Abro, Mohn, Runo, &c., a district of the Russian government of Livonia, lies across the mouth of the Gulf of Riga, between 57° 55' and 58° 40' N. lat., has a length from south by west to north by east of about 45 miles, and according to Strelbitsky contains an area of 1010 square miles. It is separated from Courland to the south by the Strait of Dosmesnes, from the island of Dago to the north by the Sola Sound, and from the smaller island of .Mohn on the north-east by the Little Sound. Its undulating surface, well watered by numerous streams, and consisting mainly of clay and disintegrated limestone, is largely occupied by woodland and pasture, but also allows the cultivation of considerable quantities of grain, flax, hemp, and potatoes. The coasts are bold and steep, and, especially towards the north and west, form precipitous limestone cliffs. Like those of Shetland, the Oesel ponies are prized for their smallness. The population, numbering 50,566 in 1870, is mainly Protestant in creed, and, with the exception of the nobility, the clergy, and some of the townsfolk, Esthonian by race. The chief town, Arensburg (Esthonian, Kurre Scare), on the south coast at the mouth of the Peddus, is a place of from 3000 to 4000 inhabitants, with two churches (a Greek and a Lutheran), two hospitals, an orphanage, and a trade in grain, potatoes, whisky, and fish.

Oesel at an early period belonged to the Teutonic knights, and was governed by its bishops till 1561, when it passed into the hands of the Danes. By them it was surrendered to the Swedes by the peace of Briimsebro (1645), and, along with Livonia, it was united to Russia in 1721. Arensburg, dating as a castle from the 14th century, became a town in 1563.

(ETA. See THESSALY.