Oahu, island in central Hawaii, between Kauai and Molokai islands, in Honolulu County; it is the third largest island in the state, after Hawaii and Maui. The most heavily populated of the Hawaiian islands, Oahu is known as the Gathering Place. It is about 64 km (about 40 mi) long and 42 km (26 mi) wide. Oahu is the commercial center of Hawaii and is important to United States defense in the Pacific. Pearl Harbor naval base is situated here. Two parallel mountain ranges, the Koolau Range and the Waianae Range, cross Oahu from northwest to southeast. A wide fertile plateau separates the two ranges. The chief agricultural industries in Oahu are the growing and processing of pineapples and sugarcane; tourism also is very important to the economy. Among the many popular beaches is the renowned Waikiki, backed by the famous Diamond Head, an extinct volcano. The largest community, Honolulu, is the state capital.