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I was on the Oriana (P&O) last year and found the evenind dress code was smart formal and for the 2 formsl nights add a glitter wrap & gold?silver shoes and enjoy. There were the what I would call 'overdressed' but really smart is enough. Relax and be comfortable.
Smart casual you can't beat the Ocean Village it is more casual.
I would reiterate what Jackson says. I (53) went on a two week cruise with P&O on the Oceana, last July with my wife (50) and my twin 21 yr old daughters. I hate dressing up so wasn't looking forward to the black tie nights. But I dutifully hired a tux for the occasion and took a suit with me for the semi formal nights and I thoroughly enjoyed the whole thing. I found that, even on the nights when I could dress down a bit, I chose to dress a little smarter than I might otherwise have done as I don't get much opportunity at home. The ship, in the main was full of ordinary people just out to have a good time and not a drunken hooligan in sight. Of course in the daytime it' shorts and t-shirts as usual.
You can, if you want just wear a dark suit on the formal nights but most men do wear tuxedos or dinner jackets and ladies just wear whatever they feel glamorous in.
You will enjoy it even if you try hard not to.