I hate to break the news to you, but Disney does not close it's restaurants (at least the most popular ones anyways - and O'Hana's falls into this category) for private parties and events during regular dinner hours. This is a huge "No-No" and is referred to internally as a "major guest disatisfier."
(Imagine if you'd saved for five years just to bring your family to Disney for their dream vacation and you've heard about the amazing dinner at O'Hana's. Then, on the last night of your vacation, you show up at O'Hana's for your special "last Disney meal" only to discover that it's closed for a private party. This is what's known as a MAJOR GUEST DISATISFIER! It doesn't matter how much you are willing to pay to close the restaurant for a private party, Disney does not like to upset a large number of other guests in the process so they won't do it.)
I know that example was a bit dramatic, but it gets the point across.
Now, with that said, you could always look into reserving O'Hana's privately (or other Disney restaurants) in the afternoon before they open for dinner (5 p.m. is the standard opening time.) Additionally, several Disney restaurants offer "private rooms" specifically for this purpose. California Grill, Jiko, Spoodle's and Citrico's are a few of the Disney restaurants that offer private and semi-private rooms that can be utilized during regular dinner hours.
Or, you could look at doing a private event in a Disney restaurant after park close. A good example is doing something at the Brown Derby restaurant at the Disney-MGM Studios. Certain times of the year (during "off-peak" season), the Studios closes at 7 p.m. On these nights, it can be possible to begin a private party in this restaurant about 2 hours after park close (so 9 p.m.). Of course, this would mean a later dinner, but think about how fabulous it would be!
In addition, the Living Seas at Epcot offers a beautiful VIP lounge that is very similar in look and feel (and size) to the Coral Reef Restaurant, which is situated just below it. The Living Seas Lounge is available anytime day or night.
Just contact your Disney Event Manager and they can tell you about all the group dining options. Good luck!