It's warm on Oahu, so pack your summer attire. You may want to bring a light jacket or sweater for the evenings. Suits and ties are rarely worn here. Bring some casual dress clothes or resort wear if you plan on experiencing Oahu's nightlife. You can buy an Aloha Shirt and flip-flops (or slippers, as the locals call them) when you get here.
Consequently, besides your driver's license, credit card, camera, binoculars, and other essentials, it's best to keep your clothes to a minimum... a jacket, sweater, one or two pair of washable slacks/shorts, walking shoes, sandals and swim gear. Sunscreen is essential since Hawaii's close proximity to the Equator translates into very strong sun radiation. The suitcase space you save can be used to fill up on island purchases.