Oahu (ōäˈhō) [key], island (1990 pop. 836,231), 593 sq mi (1,536 sq km), third largest and chief island of Hawaii, part of Honolulu co., between Molokai and Kauai. Oahu is composed of two parallel mountain ranges (Waianae and Koolau) that are separated by a rolling plain dissected by deep gorges. Mt. Kaala (4,040 ft/1,231 m) is the island's highest peak. Oahu has no active volcanoes, but there are many extinct craters, among them Diamond Head, Koko Head, and Punchbowl. Pearl Harbor indents the island's southern coast. Honolulu, the state capital and the economic center of Hawaii, is on the highly urbanized southern coast of Oahu. Manoa Valley is the site of the Univ. of Hawaii, Punahou Academy, and the Mid-Pacific Institute. The island is an important defense area that includes the headquarters of the U.S. Pacific Command and the Pearl Harbor naval base. There are many bathing beaches (including Waikiki), some of which have coral gardens. Large pineapple and sugarcane plantations cover the rural areas of the island, and their products form Oahu's chief agricultural exports. Dairy farming and fishing are also important activities, but tourism is the principal economic mainstay of Oahu.