The 
Open Sound System is the "old" 
sound card Linux kernel driver system which was used in the kernel up to version 2.4.x. Starting with version 2.5 of the kernel, the OSS is replaced by 
ALSA, the 
Advaced Linux Sound Architecture. ALSA has a compatibility layer that enables old applications to work under 2.5 and forth.
The Open Sound System is not invented by the Linux kernel developers but by the company 4Front Technologies[?], a company that also sells a proprietary version of OSS for other UNIX variants, such as Solaris.
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