Whilst there are special events during the Christmas cruise (i.e. Captain's introduction cocktail party) none of these require black tie or evening dress. Dress code throughout is casually elegant - much as you woudl expect in a 5-star country hotel.
Hang on a minute we have, once more, got a confliction and there is no point in answering the question until we know whether you are talking about Oceania Cruises as the bottom line says or the P & O ship Oceana mentioned in your Query. Please elucidate... Neil.
I am speaking of the cruise line..oceania in particular...regatta.