Oahu is the best of all the Islands for nightlife. The locals call it pau hana, but you might call it happy hour (the literal translation of the Hawaiian phrase is "done with work"). On weeknights, it's likely that you'll find the working crowd, still in their business-casual attire, downing chilled beers even before the sun goes down. Those who don't have to wake up in the early morning should change into a fresh outfit and start the evening closer to 10 pm.
On the weekends, it's typical to have dinner at a restaurant before hitting the clubs around 9:30. Some bar-hoppers start as early as 7, but partygoers typically don't patronize more than two establishments a night. That's because getting from one Oahu nightspot to the next usually requires transportation. Happily, cab services are plentiful, and rideshares like Uber and Lyft give Honolulu a San Francisco feel.
You can find a bar in just about any area on Oahu. Most of the clubs, however, are in Waikiki, near Ala Moana, and in Chinatown, near downtown Honolulu. The drinking age is 21 on Oahu and throughout Hawaii. Many bars will admit younger people but will not serve them alcohol. By law, all establishments that serve alcoholic beverages must close by 2 am, although you might get lucky and stumble into a secret all-night party. The only exceptions are those with a cabaret license, which can stay open until 4 am. Some places have a cover charge of $5–$10, but with many establishments, getting there early means you don't have to pay.