Oceania has much smaller ships. It is supposed to have excellent food. While I haven't been on it yet--we're booked for our first cruise in February--I have been on Celebrity several times though not in the past 10 years, and while its food was okay, my understanding is that it may have gone downhill somewhat. Also, Oceania's service is supposed to be excellent. I can't say Celebrity's was anything special. However, I'm not expecting a Regent experience on Oceania either. Most people categorize Oceania between lines such as Celebrity and the luxury lines.
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Eugene