Judy and John,
Yes, we love Oceania. Actually, love the ships ( we used to take Renaissance all the time -because the small ships- so have been on them alot ). They have been made even more gorgeous since Oceania bought them. As far as the line have been on them twice since they started last year and going to South America in Feb. They do have excellent itineraries and wonderful food. What more do you need? lol. They have 3 speciality restaurants( no additional charge)- like Radisson , and a main dining room. An eat when and with whom you choose dining policy.. so again the same.
None of that pre arranged by ship dinner friends
( unless you choose them wink. They have a more relaxed dress code .. no formal nights but that doesn't mean that people don't dress up- they do.. just no ball gowns and tuxes, men can choose to wear sports coat & ties to dinner or not. They call it country club casual - I would say that means what you'd wear to dinner any other night ( other than a formal night on Radisson).. so no difference there.
On Oceania, the prices have been really great too with many cruises getting two for one ( like Radisson ) and free air thrown in or if you want to get your own air you can take their air credit on some itinys so that is good. It is not all inclusive as Radisson , wine and gratuities thrown in, but I think if you compare the penthouse suite on O with R for 14 day itinerary you will be paying so much more on R so suppose it all works out.
I haven't been on the Diamond so can't help you with that comparison, but have been on Mariner and loved it. I think this ship would be a great comparison to that or Navigator and one for you to try if they have the itiny that you want to do. We loved Istanbul so I know how you feel about missing that port. By the way,
you do get butler service with all cabins penthouse and above and the penthouse rooms are really spacious. They just announced some really interesting ports for next summer ( only other ship going to places like that Tunisa, Libya) is Silversea. Interesting.
Well if you have q's about the ship just ask.