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firstly the old sunbed question.It is extremely unlikely that you can reserve a sunbed, and in fact most cruise lines take the view that if a bed is left unattended for longer than 30 minutes then they will remove towels etc.
They are trying to be fair as there's nothing more annoying than you wanting a sunbed and seeing one with a towel on all day. Sunbeds tend to be busier on a sea day, so get on deck a little earlier if you can and you should have a good choice.
On the times I have been on a P&O ship in warmer climes, they do tend to put on a bbq or buffet night, perhaps even a specialised meal night. Your husband will be fine in a shirt and trouser. Hope this helps and Im sure you'll have a great time as Oceana is a lovely ship. David Powell travel journalist.