Oceania ships are smaller and have the ability to get into ports that the Carnival (and other lines) cannot. The R Class ships are roughly 30,000 GRT with roughly 650 pax on them. Take Bermuda for example, Oceania will port right in downtown St George, and run over to Hamilton. With the larger ships you lose this ability.
For what it is worth, my avatar photo is from an Oceania sailing to Alaska.
DW and I have sailed on HAL, NCL (Haven), Oceania, MSC (Yacht Club), and Carnival. Each cruise line has plusses and minuses. If you are a more destination focused cruiser, Oceania will be more your cup of tea. But just be advised, casual for them is slacks and a polo (tshirts are discouraged). I usually dress like this anyway, so it does not bother me.
We just got back from a 10 day sailing on Carnival, so we do bounce around.