We did one Oceania and made a list of pros and cons. Overall we decided to stick to Celebrity.
O has much nicer included drinks on their drinks package and you can get them via room service, too. The food is better as well but if you keep an odd schedule there are times when nothing is available but room service. Also, while the quality is better, the selection is smaller because fewer passengers.
There are not nearly as many activities on O, so you'll mostly be eating and relaxing, maybe gambling in the very small casino.
You run into the same passengers over and over which is great if you like to socialize. Everything is open seating.
There is a lot more wave movement, if you get seasick easily.
You'll have to consider what is important to you and go from there.