Oahu or Oahu , known as "The Gathering Place", is the third largest of the Hawaiian Islands; however, it is the most populous of the islands in the U.S. state of Hawaii while also having the primary and only intercontinental Honolulu International Airport. 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 , making it the 20th largest island in the United States. In the greatest dimension, this volcanic island is long and across. The length of the shoreline is . The island is the result of two separate shield volcanoes: and , with a broad "valley" or saddle (the central Oahu Plain) between them. The highest point is Mt. Ka'ala in the Waianae Range, rising to above sea level.
The old Kingdom of Oahu was once ruled by the most ancient in all of the Hawaiian Islands. The first great king of Oahu was Mailikukahi, the law maker, who was followed by many generation of monarchs. Kualii was the first of the warlike kings and so were his sons. In 1773, the throne fell upon Kahahana, the son of Elani of Ewa. In 1783 Kahekili II, King of Maui, conquered Oahu and deposed the reigning family and then made his son Kalanikupule king of Oahu. Kamehameha the Great would conquer in the mountain Kalanikupule's force in the . Kamehameha founded the with the conquest of Oahu in 1795. Hawaii would not be unified until the islands of and surrendered under King Kaumualii in 1810. Kamehameha III moved his capital from Lāhainā, on Maui to Honolulu, Oahu in 1845. , built later by other members of the royal family, is still standing, and is the only royal palace on American soil.
Oahu was apparently the first of the Hawaiian Islands sighted by the crew of HMS Resolution on January 19, 1776 during Captain James Cook's third Pacific expedition. Escorted by HMS Discovery, the expedition was surprised to find high islands this far north in the central Pacific. Oahu was not actually visited by Europeans until February 28, 1779 when Captain Charles Clerke aboard HMS Resolution stepped ashore at Waimea Bay. Clerke had taken command of the ship after Capt. Cook was killed at Kealakekua Bay on February 14, and was leaving the islands for the North Pacific.
Today, Oahu has become a tourism and shopping haven. Over five million visitors (mainly from the American mainland and Japan) flock there every year to enjoy the quintessential island holiday experience.
An earthquake, measuring 6.7 on the Richter scale, struck the Island Of Hawai'i and the surrounding islands at 07:07:49 HST on October 15, 2006, causing an island-wide power outage and over $200 million in damage.