On Board P&O there are a range of formal, informal and smart casual evenings. In the formal nights ladies are expected to dress up and wear formal evening and cocktail outfit. In the informal nights ladies have the option of tailored trousers. on the smart casual nights you can wear casual separates e.g. trousers, casual tops or a dress.
In my experiance of formal nights on p&o ladies wear either long or short dresses,I would advise that if you are comfortable,and feel good,then you look good.Everyone does make an effort on formal nights and all look smart, which is really lovely.