It is version 1.2, it is described on these web pages and in the OMDoc specification.
The development of OMDoc 2.0 has already begun, this will be a general (backwards-incompatible) overhaul of the document format, that will take all the experiences with OMDoc 1 into account and track all the developments of web standards since OMDoc 1.2. The development proceeds in steps; see the Development Plan for an overview and roadmap for a progress report in terms of GitHub tickets.
the OMDoc format is mainly intended for machine consumption. You do not want to look/edit it in this form. For viewing, transform it into html or LaTeX (see the [ToolsSystems tools] section, for editing use an OMDoc or xml editor.
If you are concerned about disk space and bandwidth, consider using xml-specific compression tools like xmill or xmlppm. They take document structure into account during compression and very often realize compressed files that are smaller than the non-xml/OMDoc format of the data.
##How do I edit an OMDoc document? You have the choice of either
nxml-modeis a general XML mode that supports RelaxNG schemata. There is also a dedicated OMDoc mode that needs some love and affection.
Yes, we do; see details here
There are multiple possibilities, you can