Oahu /oʊˈɑːhuː/ or Oʻahu (pronounced [oˈʔɐhu] in Hawaiian), known as "The Gathering Place", is the third largest of the Hawaiian Islands and the most populous of the islands in the U.S. state of Hawaii. The state capital, Honolulu, is on Oahu's southeast coast. Including small close-in offshore islands such as Ford Island and the islands in Kaneohe Bay and off the eastern (windward) coast, it has a total land area of 596.7 square miles (1,545.4 km²), making it the 20th largest island in the United States. In the greatest dimension, this volcanic island is 44 miles (71 km) long and 30 miles (48 km) across. The length of the shoreline is 227 miles (365 km). The island is the result of two separate shield volcanoes: Waiʻanae and Koʻolau, with a broad "valley" or saddle (the central Oʻahu Plain) between them. The highest point is Mt. Ka'ala in the Waiʻanae Range, rising to 4,003 feet (1,220 m) above sea level.

The island is home to about 953,207 people (approximately 75% of the resident population of the state, with approximately 75% of those living on the "city" side of the island). Oʻahu has for a long time been known as "The Gathering Place". However, the term Oʻahu has no confirmed meaning in Hawaiian, other than that of the place itself. Ancient Hawaiian tradition attributes the name's origin in the legend of Hawaiʻiloa, the Polynesian navigator credited with discovery of the Hawaiian Islands. The story relates that he named the island after a son.