Duke's Waikiki at the bar. Afternoon to early evening is the best, especially when they have someone playing live music. Lots of women there, especially Sunday afternoon. The yardhouse is a restaurant/bar where people go as well, I like Duke's a lot better since its right on the water and is more chill/hawaiian in vibe.
Most of the tourist nightlife is right in Waikiki so I suggest staying there. There are a few University of Hawaii dorms in the area, but I find going to tourist spots a little better as the college bars are...well, college bars.
Lots of the nightlife tends to be on the messy/ghetto side. Lots of locals keep to themselves and are extremely clique-y. There are a few ok irish/english pubs, but stuff ends pretty early out there, its a lot better to meet people during the daytime. Alot of people go there on vacation or honeymoon, so take that into consideration.
Take a booze cruise too. They're not too expensive and you meet people there who are out to drink, hang and have a good time. They're just right on the beach in front of dukes, several of them. Only like $35 for unlimited drinks and a 2 hour cruise around Waikiki on the catamaran and you can jump in the ocean.
I go to Oahu every year.