Ofu and Olosega are parts of a volcanic doublet in the Manu‘a Group of the Samoan Islands—part of American Samoa. The twin islands, formed from shield volcanoes, have a combined length of 6km. They are geographic volcanic remnants separated by the narrow 137m wide Asaga strait, a natural bridge of shallow coral reef. Before 1970, one had to wade between the two islands at low tide; now a single-lane road bridge over the strait connects villages on Ofu island with those on Olosega.
The island forms the Olosega County subdivision of the Manua District. It has a land area of 5.163 km² (1.993 sq mi), and had an official population of 216 persons as of the 2000 census.