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 6 km and an area of 12 square kilometers (5 square miles); their population is about 500 people. They are geographic volcanic remnants separated by the narrow 137-meter-wide (449-foot) 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 highest peak on Ofu is Mount Tumutumu (491 m (1,611 ft), also referred to as Tumu) and the highest elevation on Olosega is Mount Piumafua (629 m (2,064 ft)). The most recent volcanic eruption took place in 1866, 3 km (1.9 mi) south east of Olosega.
Archaeology field work carried out in the 1980s yielded pre-historic evidence including ceramics, adzes, shell and bone which have been significant in furthering understanding of the ancient history of the Samoa Islands and Polynesia. This included samples of red-slipped plainware ceramics that appeared to be in the tradition of the Lapita culture. The work, carried out by a team that included Pacific archaeology specialist Patrick Vinton Kirch, focused on a site called To'aga (site AS-13-1), a 2 km (1.2 mi) coastal stretch on the south coast of Ofu. The results showed continuous human habitation of about 3,000 years.
Ofu is the western part of the volcanic outcrop of Ofu-Olosega Island. The main village of Ofu is located on the western shore, protected behind an offshore islet (eroded tuff cone) known as Nu'utele. Ofu has a small airport (IATA: OFU, ICAO: NSAS) and a boat harbor that serve the population on Ofu and Olosega. Before regular airline service was discontinued in 2009,[why?] the flight from Pago Pago took about half an hour.
Most of the southern shore and associated coral reef are part of the National Park of American Samoa. In 2005 the U.S. National Park Service was negotiating with village councils on Olosega to expand the park around that island.
Situated on the south coast of the island is To'aga lagoon which has a high diversity of corals and fishes. The marine site has been part of long term research and study on coral reefs and global climate change.
The island is also home to the Samoa Flying-fox (Pteropus samoensis), a species of bat threatened by habitat loss.