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P&O is not very formal these days and for most nights smart casual dress is perfectly adequate. They may have one or possibly two formal nights and for these simply go to their alternative dining such as the buffet. Most important thing is to be yourselves and just enjoy yourselves because the days of great formality have gone and it now focused very much at the mass market and by the end you may surprise yourselves and wish it was a little bit more formal.
On a one week cruise there are normally two formal evening.