We were on the same trip out of Venice (disembarked in Barcelona) and like you were first time Oceania. We booked a PH1. Also like you could not find fault with Riviera and the crew. The ship is beautiful, tastefully decorated and very stylish. The crew were amazing and we can only sing their praises in regards to friendliness and service from the butler to housekeeping to food & beverage staff, even the maintenance team painting around the ship were always pleasant and chatty.
I'm glad you mentioned some of the behaviour at the buffet. We found a lot of very rude and arrogant people in here, pushing in front of you, speaking over the top of you when you were been served and frankly extremely rude the way they treated and spoke to the friendly staff serving them food. In fact it put us off eating in there several times !!
Overall I can't fault our first Oceania.