You can go to the Plaza or you could also go to Cafe Jardin. Both are informal.
There is a self service rest you can use, food is very good, waiters come around and ask if you want a drink, water is available from machines fre.
Yes the Oceana has introduced freestyle dining, plus you can use the Cafe Jardin, check your brochure out at the back in the question and answer section.
Yes there is always an informal alernative restaurant if you don't want to dress for dinner, if you opt for anytime or as P&O term it freedom dining between the hours of 18:00 to 22:00 one still dresses for dinner on formal nights.
It ia worth noting that even if you choose freestyle dining - freedom dining it is called - you will still be expected to adopt the dress code of the evening. Also if you want to get a drink in any of the bars the same applies. There are now notices outside the bars and restaurants stating this. We were on Oriana recently and someone was told they could not be served in two of the bars because he did not have a jacket on when it was an informal evening. In the Caribbean you will have 4 formal evenings and the rest smart casual. You will find that the ship takes on a really glamourous feel on the formal evenings. Men can wear a suit if they prefer on a formal evening although I would say about 96% of men wear dinner jackets.