Yes there was a Sushi bar on the Ocean, it was 9/00 during lunch the Sushi bar was located outside the buffet resturant, it was not every day only twice during a 14 day cruise,so you will have to watch for it, I don't even think it was listed in the platter. Make sure you get the large shrimp they have at the Sushi bar, they don't last long
Princess usually offers a featured presentation in the buffet restaurant ("Horizon Court" aboard MV Ocean Princess) at lunchtime on a day when the ship is at sea. On my last cruise ("Scandanavia and Western Europe" aboard MV Royal Princess), we had four days at sea and four different featured presentations -- one of which was a Chinese buffet and Sushi Bar (officially called the "Golden Dragon Buffet & California Sushi). The other featured presentations were the "Pie in the Sky Buffet" (sorry, but I don't recall the details of that one), the "Symphony from the Sea Buffet" (obviously featuring seafood), and the "Dolce Vita Luncheon Buffet" (obviously featuring Italian cuisine). The sushi bar seems to be fairly common on cruises with fewer full days at sea, but is not necessarily guaranteed.
All of the new Princess ships have a wine and caviar bar ("Apres-Vous" aboard MV Sea Princess) where you can enjoy caviar any evening (at an extra charge). Additionally, the main dining rooms offer caviar as an appetizer on formal evenings at no additional charge.