Part of Ogg Vorbis tools. ogg123 (obviously taking its name from mpg123) is a program used to decode Ogg Vorbis audio on command line.
It normally uses libao to output to the soundcard - that is, ogg123 music.ogg plays the music file. It supports OSS, ALSA, Sun and IRIX sound devices, and EsounD and aRts sound daemons.
There are different file output drivers, though. Like mpg123, it can decode to a .wav file, and also .au and .raw files.
An example on how to decode .ogg to .wav:
ogg123 -d wav -f music.wav music.ogg