We will never be able to provide 100% OSS compat in PA -- and not even 100% ALSA compat.
Also, note that the OSS compat ALSA in the kernel provides is far from perfect. Doesn't support userspace plugins -- it hence is incompatible with userspace plugins, such as dmix, and everything else.
Also, OSS as an API is broken in the timing model. The timing model is so broken that it desn't really work properly on USB audio.