The Oasis is massive but being a big ship has its pros and cons. The best things about the Oasis are the shows, activities and facilities. The rooms were bright, new and clean. The open areas were expansive, wonderfully decorated and unique. Even though there were 6000 passengers on the boat, the only time that we ever saw crowds were at the shows. The shows were hands down the best part of our cruise. There was a different show every night and they were all professionally done. I felt like I was on Broadway in NYC. To make your trip smoother, make sure you reserve all of your showings online before the cruise. Also, there is no need to get there 30 minutes early to find a seat - usually 10 minutes is more than enough. We had a oceanview balcony room and highly recommend it! Given how big the ship was, if you did not seek out the ocean, you can easily forget that you were sailing in it. Waking up to the ocean views was priceless especially when having room service breakfast on the balcony. Our favorite part of the ship was the Central Park because it was a serene escape from the noise and madness of the parties held throughout the boat.
Now onto the bad.
Having to service 6000 passengers means that the service is subpar. During dinner, we had to ask our waiter repeatedly for items. We never saw our headwaiter (and wished we didnt tip her) and our assistant waiter always looked exhaustive and checked out. The food was much worse than on the 4 other Royal Caribbean cruises that we have been on - most of the hot food was served cold and the meats were overcooked. Avoid the Windjammer during breakfast - the crowds are crazy and it was like a bazaar. By 9:30am I had to scratch the side of the pot for any remaining oatmeal. Eat breakfast at Opus Dining Room when it is open (make sure you go to the a la carte and not buffet side side), Park Cafe or anywhere else. The speciality restaurants were okay. Giovanni's is worth trying but the food at Izumi was not good. Everyone got their entrees at different times and the service was slow. Have lunch at the Dining Room when it is open as well - the salad bar is amazing.
Be careful of the Flowrider! I took a private lesson at night and when I did the surfing, I "wiped out", hit my head on the back of the board and almost had a concussion. When I went to the medical center they said that they have 5 accidents a day on the flowrider - some guy got foam embedded his nail and another broke his finger. If you must go on, make sure you learn how to fall correctly.
Try to get the sailing that does not go to Mexico. Cozumel and Costa Maya are very run done. There is nothing to do except for a shore excursion that involves snorkeling or scuba diving. We have been to Mexico a lot and definitely do not feel that these 2 ports give a good you a good taste of Mexico