I'm on Oosterdam right now and the nightly shows are still there! (Well, it's only day 2 or 3 but so far...) I'm not sure if masks are required in the theater because I haven't gone to a show yet.
If you choose to wear masks, you will be in the minority. But I haven't seen anyone being hassled about it.
If you want to have dinner at your own table, they can generally do that in the MDR but it might be a few minutes more of waiting versus sharing a table... maybe 10 minutes more, if that. (Excluding gala nights or whatever the new name is...) You might want to make reservations for a table for 2 those nights, or every night. You can do this even if you have the flexible dining option (i.e., no fixed dining time).
I've only seen a handful of people wearing masks on the ship. On the excursions, you're supposed to wear a mask inside the bus but I don't know how well this is enforced. On the bus ride from Venice to Trieste, only about 4-6 people out of approx. 50 wore masks. There wasn't even an announcement to say masks were required or recommended.
I feel safer than at my home but we have a lower vaccination rate. I don't believe you can ever be 100% safe but the ship is not full and there are plenty of places to go where you're able to sit farther away from others.