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You can contact P&O and ensure you have been put on the wait-list for Club Dining. In order to get Club Dining you usually have to book very early. When you get on board there will be a card in your cabin telling you which dining you have been allocated. If you still haven't been allocated Club then go down to the restaurant - I think from 3p.m. - and you will find the head waiter there who will do his best to accommodate you. Good luck.
We've just come back from a great 2 weeks on Oceana. We were in the same position as you, but it worked out fine:
1. Pretty much on day one we went up to the Head Waiter and switched to Club Dining with no fuss
2. After a few days we met a very nice couple who we fancied dining with, so we queued up for Freedom Dining..we were waiting for no more than 10 mins. Seemed to run fairly smoothly for us.
3. One or two nights we visited the Plaza (sometimes for "second desserts"!!)
So, don't worry - you should be fine.