I am scheduled for the Allure at the end of April. Other topics in this forum have assured me that as long as I show up early for a show, I will get into it, because many people abandon their reservations. The only exception is the Comedy Club -- that one is hard to do a "walk in", apparently. But no worries on the other shows.
I feel sure that you will get a more authoritative answer from someone who has sailed the Oasis recently, but I wanted to assure you that all is not lost! Have a great cruise!