OESEL (in Esthonian Kure-saare or Saare-ma)  , a Russian island in the Baltic, forming with Worms, Mohn and Runo, a district of the government of Livonia, and lying across the mouth of the Gulf of Riga, 1o6 m . N.N.W. of the city of Riga . It has a length of 45 m., and an area of Toro sq. m . 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 small, but prized for their spirit and endurance . The population, numbering 5o,566 in 187o and 6o,000 in 1900, is mainly Protestant in creed, and, with the exception of the German nobility, clergy and some of the townsfolk, Esthonian by race . The chief town, Arensburg, on the south coast, is a place of 4600 inhabitants, with summer sea-bathing, mud baths and a trade in grain, potatoes, whisky and fish . In 1227 Oesel was conquered by the Knights of the Sword, and was governed by its own bishops till 1561, when it passed into the hands of the Danes . By them it was surrendered to the Swedes by the peace of Bromsebro (1645), and, along with Livonia, it was united to Russia in 1721 .