This package ensures that Open Sound System support is provided in some way. On Linux, it enables the ALSA compatibility modules. On other kernels where OSS is the default interface, no action is taken.
The purpose of this package is for applications that only support OSS to depend on it, hence preventing common "/dev/dsp not found" errors that would confuse unexperienced users.
On Linux platforms a better approach nowadays is to use osspd, which emulates OSS devices using ALSA or PulseAudio.