There is only one file type that uses the .MO3 extension:
MO3 Audio File
Modular audio file format that is similar to other MOD formats, such as .MOD, .XM, and .S3M; however, MO3 files contain samples compressed with .MP3 or .OGG audio compression and also use a compressed file structure; allows higher-quality samples to be used while significantly reducing the file size.
OGG-encoded MO3 files can be created with OGGENC. MP3-encoded MO3 files can be generated using Rarewares LAME software.
Updated: December 4, 2013