In my experience on Oceania cruises, they do an excellent job of catering to dietary restrictions. If you notify guest services of your needs in advance of the cruise, they will provide you with menu selections/preparations that meet your needs a day in advance for you to make selections. The maître d’ in the dining venue will have a written sheet to hand to your waiter when you are seated. They are accustomed to adjusting meals for passengers when given sufficient notice, and are gracious about it. Enjoy your cruise.