O’ahu is the third largest and most populated island in the Hawaiian island chain and home to the Hawaii State capitol, the city of Honolulu. Honolulu is the business and financial center of O’ahu, as well as home to Hawaii’s major international airport, major seaport, and State University. The entire island of O’ahu is Honolulu County in the State of Hawaii.
Approximate Population of O’ahu: 
Approximately 909,000, according to 2006 census data.
Approximate Population of Honolulu:
Around 380,000 (46th most populated city in the United States). Also, over 4.7 tourists visited in 2006.
Approximate Size of Oahu:
Around 600 square miles (O’ahu is smaller than both the Big Island of Hawaii and Maui).
Geology of O'ahu:
O’ahu is the second oldest island of the Hawaiian chain. It was formed by the eruptions of two shield volcanoes, the Waianae and Koolau, after which the respective Mountain Ranges were named, bridged in the middle by the Central O’ahu Plain. The highest point of O’ahu, Mt. Ka’ala is 4,003 feet above sea level.
Oahu’s Climate:
Like most of the other Hawaiian islands, O’ahu has several micro-climates. In general, the Windward (E/NE) side of the island is windier and rainier than the Honolulu area. The mountain areas are the rainiest areas of the island and the Leeward side of the island is the hottest and driest. And, as you would guess, the areas with the most rain are the most green and tropical. The drier areas are more desert-like, some areas even having cactus! Most of the rain on the island falls in the winter months.
The average temperature on O’ahu is between 75-85 degrees F. As a practical matter, there are really only two seasons on this island: Summer (Kau), May-October, and Winter (Holo), November-April. On the hottest days of summer, temperatures may go from a low of about 78 degrees F, to a high of about 92 degrees F. The coldest winter temperatures may go from a low of about 66 degrees F to a high of about 80 degrees F.
Time Zone for O'ahu:
Hawaii Standard Time (GMT -10 hours), which is 5 hours behind EST (e.g., New York) and 2 hours behind PST (e.g. San Francisco), but add another hour to each of these during Daylight Savings Time because Hawaii does not observe this change.
Language Spoken on O'ahu:
American English
Though, you will hear many other languages spoken frequently (Hawaiian slang, Japanese, Tongan, Chinese, Korean, Portuguese, etc.) SEE ALSO, My Hawaiian Words Page
Ethnically Hawaiian (Hawaiian or part-Hawaiian) people make up less than 20% of the population of O’ahu, which is a truly a melting pot of Asian and other cultures. Hawaii has seen waves of immigration from many countries and regions including the following:
As a result, the Caucasian population of the island is just over 20%, Japanese - around 18%, Filipino - around 14%, Chinese - around 5%, and Tongan/Samoan - around 2% and Black and Korean – each, around 1%. The mixed race population of the island is just around 20%
O'ahu's Currency:
US Dollar
Note: Hawaii may seem like a foreign place to some, since it is overseas. But Hawaii is the 50th state of the United States of America, and US citizens here have all the same benefits and protections of US citizenship here, as they do on the mainland. US citizens do not need a passport to travel here.
O'ahu's Major Industries:
Travel/tourism, US military/defense, construction, government, manufacturing, agriculture.
Oahu’s Nickname: 
The “Gathering Place”
Oahu's Flower:
Ilima
O'ahu's Color:
Yellow (mele mele)
Air: Honolulu International Airport (HNL)
Land:
H-1 – Goes across Southern to mid section of island East to West.
H-2 – Goes from Central Oahu to Northshore
H-3 - Goes from Central Oahu to Kailua/Kaneohe
H-201 – Moanalua Fwy - connects Honolulu to Halawa Heights/ Aloha Stadium/ Camp Smith
Pali Highway (61) – Goes from Downtown Honolulu to Kailua, through the Ko’olau Mountain Range
Likelike Highway (63) – Goes from Honolulu (Kalihi/Chinatown area) to Kaneohe, through the Ko’olau Mountain Range
Kalanianaole Highway (72) – Goes from East end of H-1 (Kahala area) around S/SE part of O'ahu to Kailua
Kamehameha Highway - (99 ) Goes through Central Oahu from Hickham/Pearl Harbor through Aiea and Pearl City, heading North and then around the North Shore, through Kaneohe and ending at the Pali Highway, near Kailua.
Sea: Port of Honolulu.
Interesting Oahu & Hawaii Facts:
Hawaii's Legal Holidays
New Year's Day, January 1; Birthday of Martin Luther King, Jr., 3rd Monday in January; Presidents' Day, 3rd Monday in February; Kuhio Day, March 26; Good Friday, March or April; Memorial Day, last Monday in May; Kamehameha Day, June 11; Independence Day, July 4; Admission Day, 3rd Friday in August; Labor Day, 1st Monday in September; Discoverer's Day, 2nd Monday in October; Election Day, 1st Tuesday after 1st Monday in November; Veterans Day, November 11; Thanksgiving Day, 4th Thursday in November; Christmas Day, December 25.