Oriana is a very elegant ship. There will probably be at least two, possibly three theme nights - one Black and White evening (this is a Formal Night so the majority of the men will be in DJs - over 90% - and the women usually wear a combination of black and white - like a black dress with white shawl for example. There may be a Tropical or Fiesta night so bright tropical shirts and tops - with suitable trousers etc. Sometimes there is a Western Evening or 60s evening but not so many people dress for these.
I suggest you go to the P&O site and register on their Community Site. You will find a section devoted to Oriana and you will find lots of information there and you can ask any questions you have. We really enjoyed Oriana so hope you will too.