The Open Sound System is a sound architecture and collection of audio drivers for Unix/Linux systems, written by 4Front Technologies.
It exists in two incarnations, a full-featured commercial version and a Free one available under GPL, called OSS/Free. The GPLd drivers have been in the LinuxKernel since 2.0 but are nowadays being phased out in favour of ALSA. They were originally contributed to by 4Front but then evolved independently. 4Front’s website says “There is no affiliation between OSS/Free and OSS/Linux anymore except that they are both OSS compatible.”