I am a vegetarian and have eaten at Ohana three times with no problem. Generally what I do is when I make the reservation, I let the them know that there is one vegetarian in the party and they make a note of it on the reservation. I also make sure to let them know when I check in at Ohana.
There is a standard main dish they make for vegetarians as I have ended up with the same thing every time (and it was *delicious* the first two times, and kind of only ok the most recent). In addition, many of the appetizers are vegetarian.
So basically, if you let them know when you make the reservation to make a note that there is one vegetarian in the party, and then let them know again when you get to the restaurant, you should have no problems.