I haven't sailed on Oceania since about 2003, but I thought the food was good then, especially in the Lido grill. I loved sitting out on the back deck during sailaways.
We found the dining room to be pretty slow, especially at open seating dinner, unless you got there right when it opened. The service was sometimes awful but that is because with open seating you really cannot control things if you get a rush of passengers all at once.
I sailed on Renaissance and it was great as well. I tended to think not much had changed between Renaissance and Oceania, since it is almost all of the same executive staff. But now that Oceania is about 7 years old, and it has been so long, it could have changed dramatically.