Much the same as ours! Norway is on the same latitude as the north of Scotland. It can be very warm one minute and rain the next (bergen is famous for this)and it is cooler the further north you go. Take layers ie. tee shirt and jeans light cardi in bag and definately a good weatherproof anorak for cooler nights on deck and heavy rain. Remember it is sunny/light at midnight, perfect for scenic cruising. Remember walking boots/shoes or good trainers. It is beautiful, you will love it, enjoy .
P.S. Don't forget the Sun Tan Lotion!!!!!!!!!
We are on the same cruise on Oriana and really looking forward to seeing the fjords. From what I have read on the P&O site it is just as the previous answer had said. Layers seem to be the answer with something waterproof just in case. Have you booked any excursions? My husband is doing the glacier treking - but not for me! I'm sure we'll have a great time.