You can pre reserve specialty restaurant according to the number of days & cabin level
You can always ask for more reservations on the day that you want to eat in one
If you have a PH or above your Butler can serve you in your suite from the Specialties or GDR menus
https://oceaniacruises.zendesk.com/hc/en-us/articles/360005477653-How-many-dining-reservations-am-I-allowed-in-each-specialty-restaurant-
https://oceaniacruises.zendesk.com/hc/en-us/articles/360005477713-When-can-I-make-reservations-in-the-specialty-restaurant-