The Ihilani is adjacent to the Ko Olina Marina where there is at least one charter boat. The Ko Olina folks have been lousy to the locals, closing access to and tearing up the trailer boat ramp, so can't recommend a boat there. Also, one day the charter boat had to pull alongside us to let their charter take photos of the big marlin we were towing to port; couldn't catch their own. Nevertheless, if you want convenience, check with the Ihilani activities desk.
The next closest is the Waianae Harbor where there are at least a couple charter boats. There're a lot of fish, YFT and such, off Waianae in June but have no first hand knowledge of the charter boats so can't recommend one there either. See above for a first hand report and take as a caution.
As
ali'i kai says, Kewalo Basin is home to the majority of charter boats on Oahu. There are many good captains/boats there and you should search for their websites and do your own evaluation. Here's a link to just one of them:
Kewalo Charter Boat. Be sure to take a look at the boat's catches pictured in the slide show.
Another harbor with charter boats not far from the Ihilani is Haleiwa. Here's a link to just one:
Haleiwa Charter Boat. Be sure to take a look at the boat's catches pictured in their 'Reports' tab. This boat seems to do more 'shares' and as your time draws nearer you should check their 'Availability' tab to see what they've got booked.
No affiliation with any of the above boats; just trying to help a visitor to our waters. Surfing the boats' websites is a good way to get a feel for their performance. Good luck, hope you catch a big one (but not mine
).
Oahu Fishing Charters? 
