We have cruised many times with mid-size ships primarily on Celebrity, Royal Caribbean, Princess and wanted to see if we'd enjoy the "lite" luxury of Oceania. Pros: Small ship experience, no crowds. No art sales, photos & photos to buy/or not, no formal nights as found on mass-market cruises, which was very refreshing.
Country club casual is not at the high end, versus Oceania's brochure ware. Everyone dressed nicely, just not too fancy. Majority of men did not wear sports jackets to dinner, usually long sleeve shirts (no ties), and slacks. Didn't see too many fancy cocktail dresses on women either. The crew was friendly and service was attentive, even at breakfast or lunch buffet. Again, no crowds. Tours are very expensive, however, the guides and companies are better than experienced on other cruise lines.
There was a mix of passenger types as you would find on other cruise lines, most from America, with a smattering from other English speaking countries. Everyone friend.
Disappointments: Oceania is often touted as a foodie line. However, it did not live up to all the hype. My wife had high expectations because of Jacques Pépin, and we were disappointed. The food on Celebrity and Royal Caribbean is just as good. The specialty restaurant Toscana was the one bright exception! A side note: Wine by the bottle is exceptionally expensive (especially considering the wine list). If wine is important to you I'd bring on your own and pay the corking fee.
Summary: I would cruise Oceania again for the small ship experience, itinerary, and good service (not for the food).