I recently sailed on the Oosterdam and can confirm that they have vegan and vegetarian options available in the main dining room. The menu changes daily, but there will always be a vegan and vegetarian option. In addition, the Lido buffet always has vegan and vegetarian options. The chefs are usually more than happy to accommodate vegan and vegetarian requests. If you have any questions about specific menu items, the waiters and chefs can usually help you out. Additionally, I would recommend asking for a copy of the daily menu ahead of time, so you can plan ahead and make sure that you are getting the most out of your dining experience.