While everyone pointed out that OSS stands for open source software, I'd like to point out that it can also stand for Open Sound System and is often used interchangably with OSS/Free, which is at writing the most common sound driver system for GNU/Linux distributions. The kernel for linux includes many OSS/Free sound drivers (open source ofcourse!), including support for Sound Blaster, Crystal, Ensoniq, Turtle Beach and more. These drivers may be included directly into the kernel or compiled as modules. When compiled as modules, you may run lsmod (normally located in /sbin) and see the drivers as:

Module                 Size    Used by
... cut unrelated modules ...
cs46xx                54992    2 (autoclean)
soundcore              4464    3 (autoclean) [cs46xx]

soundcore is the base driver for all OSS/Free drivers and cs46xx is the driver for my particular card (a Crystal based sound card). Note how cs46xx driver is on the same line as soundcore, which portrays that the cs46xx is dependant on the soundcore driver.

Other linux sound driver systems include the commercial OSS/Linux available through 4Front, and the new and hopefully future in linux audio, ALSA, which supports more than the basic set of features supported by OSS/Free, including full-duplex audio and advanced MIDI control.