MO3 is a music file format developed by Ian Luck. MO3 files are tracker modules that contain samples encoded in the MP3 or Ogg Vorbis file formats, rather than straight PCM samples. This results in a greatly reduced file size for the module, while maintaining almost identical audio quality. Lossless audio encoding is also supported, for samples that do not compress well with lossy encoding.
The format's name is a portemanteau of MOD and MP3. Despite the name, four other tracker formats are supported besides MOD: Scream Tracker 3 (S3M), Fast Tracker 2 (XM), Impulse Tracker (IT), and MultiTracker (MTM).