OSS is practically impossible to virtualize. There are hacks like LD_PRELOAD and stuff like CUSE. But they will never be able to fully provide virtualization. Stuff like mmap audio access (which is used by far too many applications unfortunately) cannot be emulated completely. Also, the timing model in OSS is too simple to really fit on a sound server backend.
OSS as a software is dead on Linux. I really hope OSS as an API will die, too. (as soon as libsydney is more than just vaporware)
ALSA can be virtualized much better than OSS, since it has better semantics and is a shared library which supports plugins and stuff. But in the end it's an API desgined for hardware devices, and the devil lies in the details: 100% ALSA compat in PA will stay a dream.