From the ALSA wiki
The Open Sound System (OSS) is a portable sound interface which is available in 11 different Unix systems. It was created in 1992 by Hannu Savolainen of Finland.
OSS was the only supported Linux sound system used up until 2.4.x version kernels. Starting with version 2.5, ALSA was introduced, and the OSS interface became deprecated for Linux kernels.
ALSA is the recommended interface for software that is intended to work on Linux only. However, software that strives to be portable across other Unixes (including the *BSD family) typically uses OSS instead (or supports both).
See OssEmulation for ways to run OSS programs on an ALSA system.
Some of the above was taken from http://www.wikipedia.org/wiki/Open_Sound_System