When Oasis came out, there was so much attention on it, I thought for sure it was Royal Caribbean's largest and most innovative ship.
However, from looking at their website, it seems Allure and Oasis are actually pretty much the same ship with its size, neighborhoods, and open middle area.
I haven't been on the Oasis yet, but I'm really looking forward to it - especially after being on the Allure. I know the Oasis doesn't have Rita's Catina, which my wife and I really enjoyed.
I was on the Oasis a year ago on her 4th sailing, and it was the BEST ship I've been on...BUT..there were some small things that RCL changed on the Allure (moved some things around on the Board Walk and did some changes for the plugins in the rooms), and much for the better....But you can't go wrong on sailing either one....Happy Sailing....