/usr/lib/mono/2.0/OpenSystem.C.dll is in libmono2.0-cil 2.10.8.1-8+deb7u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/2.0/../gac/OpenSystem.C/2.0.0.0__b77a5c561934e089/OpenSystem.C.dll