It may not be nessecary for you at 'Ohana at all.
'Ohana is family style, meaning it is fixed menu, all you can eat, but they bring it to you. All you have to do is not eat the shrimp, or tell your server you dont want shrimp. If you can't have the shrimp even on your table because of your allergy, simply tell your server when you sit down that you are allergic and they will not bring any to you.
Lots of people don't eat one or more of the menu items at 'Ohana because they don't care for them or are allergic; I myself never touch the wonton chips (don't care for them), or the honey coriander chicken wings (great taste but wings are too much effort for too little reward for me). I make up for this by eating more veggies, potatoes, and meats - and of course that fabulous dessert!
One of the grilled meats at 'Ohana is an excellent turkey, so you could simply eat more turkey (or any of the other menu items, for that matter) and you will still walk away from 'Ohana stuffed the the gills.