Oadu, one of the Hawaiian islands, the fourth of the group in size, in Ion. 158° \V., and between lat, 21° and 22° N.; pop. in 1872, 20,671, of whom 3,129 were foreigners. The island is roughly quadrilateral; its extreme length is 33 m. and its breadth is 20 m. It is of volcanic formation and mountainous, but the highest peaks are clothed with vegetation. There are two distinct ranges of mountains, the windward and the leeward, called respectively the Konahuinui and the Waianae ranges. They exhibit few craters in perfect condition, but there are groups of tufa cones along the shore. The island is well watered, and its valleys are productive, the most fertile region lying on the northeast between Kaala and Kahuku.

Honolulu, the capital of the Hawaiian kingdom, is on the S. side of Oahu; its port is the best in the islands. It has regular steam communication with San Francisco, about 2,000 m. distant. The shores of Oahu are mostly fringed with coral reefs, often half a mile broad.