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It depends where you are on the ship, as one is at the rear, one is in the middle..they will be the same menu..
We were second sitting on our last Oriana cruise and were in Oriental..we only did lunch in Peninsular..that was March this year...perhaps later cruisers can offer a bit more detail.
They will tell you which dining room by a card in your cabin on boarding. The menu will be the same in both. Both dining rooms will have two sittings, it doesn't make any difference which one you booked.
It depends where you are in the ship..as one is at the rear, and one in the middle. There is no difference in the menu.