So if the bottom rooms are $18 a day pp, then a 7 day cruise will be $252. I'm not sure Oceania has any cruises shorter, but they might. Of course as you know $250 is the max for anyone that didn't turn Silver before Nov of 2013 I think it is.
See what I posted above. Even when you are SIlver and there is Occapp the most you're going to get in Extra OBC for the cabin is $250, with the above mentioned exception. Keep in mind this is EXTRA OBC for those that are Silver or higher. Now, if you are less than Silver any kind of paid gratuities are good, regardless of where it comes from.