The documentation of the OPmac is:

opmac-u.tex,pdf    ... user documentation (in Czech)
opmac-u-en.tex,pdf ... user documentation (in English)
opmac-d.tex        ... technical documentation (in Czech)

See http://petr.olsak.net/opmac.html for more information.

This is part of csplain package.

The version of the OPmac can be found in the file
tex/csplain/opmac/opmac.tex at line 8.
