I can't have a real opinion on Oasis or Allure until I sail on one of them. From what I have seen it looks like Oasis is not as "cramped" as some of the other mega-ships.
I sailed on Epic two weeks ago. While I loved the entertainment and the diverse dining I had a real problem with the space. To me it really felt like NCL tried to fit 45 pounds into a 40 pound sack. From the cabin, pool deck and the main areas of the ship I felt cramped. One thing that bugged me was that the height of the ceilings seemed lower than on most ships. I never took a measuring tape and measured the height but it seemed that way.
I've sailed on Voyager and I would sail it again but only with a group or the family. While it was large and had a vast array of activities for all ages it just didn't give me the feeling of being at sea. This is probably the main thing that gives me a negative opinion of mega-ships. The Epic also had that same feeling of being in a Vegas hotel rather than on a ship.
My most enjoyable cruises have been when I am on a smaller ship with a good itinerary.