I know that this is not a park restaurant so they may operate differently, but the theme parks seat until 15 minutes before closing. The kitchen stays open until all have ordered so effectively the restaurant only closes to people walking in after "closing time" but on the inside, things run as usual. O'Hana is family style so you won't have the problem of empty buffets and it is still all you can eat. If you want more they are obligated to provide you the same service as anyone else. I don't think you have anything to worry about.
As far as being rushed, IMO you are usually a bit rushed in Disney restaurants anyway. Having said that, we have eaten in parks at closing time and actually found ourselves to be less rushed to get done. I am usually mindful of the staff needing to get away so I don't mind settling the check before I am done if necessary.
I love eating at closing time. If I could manage late night meals on a more regular basis we might do it more often!