OSS and ALSA

OSS or Open Sound System is a framework for audio on GNU/Linux and *BSD systems. The GNU/Linux community eschewed OSS after improvements to the code were made proprietary, though these were eventually re-released open source.
OSSv4 is still used on FreeBSD and miscellaneous GNU/Linux distros.

ALSA or Advanced Linux Sound Architecture is the more powerful replacement framework. It is not itself a sound server like PulseAudio, it is just the Linux kernel code for handling sound as an API. PulseAudio and JACK are built on ALSA.

VLC media player supports both as separate modules: oss and alsa.

Further reading

• https://help.ubuntu.com/community/OpenSound
• https://wiki.archlinux.org/index.php/Open_Sound_System
• https://insanecoding.blogspot.com/2009/06/state-of-sound-in-linux-not-so-sorry.html