OHML is the mapping language used for interfacing MIDI controllers with OtsAV.
Documentation for OHML will be made available at around the time of the next OtsAV update release, expected shortly.
In the meantime, please look at the official OHML files bundled with OtsAV as working examples of the language.
If you need to download the latest OHM files, see the Download MIDI Hardware Control Maps (OHML) page.