Google for "linux sound tutorial". Basically, OSS was the original sound system, which now has been replaced by alsa( which has a oss emulation layer for backward compatibility). Alsa is the low level device driver. There are even higher level sound daemons in use now. For instance, Ubuntu uses the Enlightenment sound daemon, which acts as a sound server, on top of alsa. Alsa is usually built into the kernel modules, and gives basic access to the DSP, speaker, microphone, etc. There is a set of basic alsa utilities, like aplay, arecord, amixer, and alsactl, with which you can use to control the /dev/dsp.

It would seem to be simple, but sound can be complex with all the options available, like Jack, etc.

If you want one good commandline tool for conversions on linux, see Sox