Oahu's tough. You can find the 6-pack charter fleet in Honolulu just west of Waikiki area. They'll take you out for a 'good time' at the FAD's (Fish aggregation devices). If you're serious about fishing, it's probably not worth it.
IF you have a military ID, go to Kaneohe marine base. The rec. department has a number of whaler-type skiffs you can rent for next to nothing. If the weather's at all reasonable you'll have a blast trolling just outside the reefs. Follow the locals. Falling that, and if you can get onto Kaneohe, try night fishing for gant travally (ulua). To find out how, go to 'the blowhole' on the eastern tip of the island, and spend an hour or so watching to locals do it, or try to talk to someone at the rec. dept. Kaneohe.
Without military id. it'll be tough. Oahu's heavily fished and offshore's not that great. Email me for more info if you are interested.