More information about the file format and how to create it is found here:
viewtopic.php?t=46479
Keep in mind that if you're going to make this publicly available, any MIDI files that you release with this MUST be licensed to be freely distributed. This includes the following scenarios:
1. If this music is your own original creation, you need to clearly state a license for distribution;
2. If this music is an original MIDI work for somebody else, you must have permission from them to clearly distribute the work;
3. If it is a MIDI cover of a commercially-released song, you must have a license to distribute from all content creators (singers, songwriters, composers, etc.) and/or copyright holders (you're very unlikely to pay for obtaining such a license, or even be granted one if you could);
4. Sections 2 and 3 do not apply if the music is legally considered "public domain". In the US, that means it was released before 1923, but due to international law it may be more complicated than that.
None of this applies if you're only creating this file for your own personal use, but in that case, why go through the work of creating an OMB file when you can just play the files in your favorite MIDI file player on your computer or another device?