At Venusian temperatures and pressures, CO2 becomes supercritical. This means it behaves as a dense liquid yet has no clear upper bound and continuously becomes gas with height.
Can we thus say that Venus is covered with a global ocean of liquid CO2 rather than atmosphere?
It's more a question of semantics, rather than a physical reality. An ocean would probably be thought to have a clearly defined surface. On the other hand, terminal velocity is only about 5m/s, which is a moderate running speed (Usain Bolt does 10 m/s). This gives you an idea about how thick the gas really is. In fact, one of the Pioneer Venus probes kept sending even after impacting the surface without a parachute or any other kind of landing equipment.
I would say the question is fair to ask, but if you consider the picture an average person has of an ocean, calling it an ocean would mostly cause confusion.
Venusian lower atmosphere qualifies as a supercritical fluid, but there isn't any clear liquid-gas interface (a surface layer) where Venusian ocean and its atmosphere would come in contact with each other. Actually, this dense lower atmosphere that reaches supercriticality due to pressure and temperature only make this boundary layer less clearly defined.
So it's up to your definition of an ocean if you still want to call the lower atmosphere of Venus like that, but I haven't noticed it called an ocean in any planetary science literature, unless they discussed Venusian geologic evolution and potential that it could have once had large bodies of water. And the lack of clearly definable surface, in my view, qualifies as a good reason not to.
Note that even if you still want to call it an ocean, this lack of clearly defined boundary layer also means that you'd have problems establishing its volume.
