OEM encoding is used on Windows by the console (.bat, .cmd files). If a string contains diacritical characters, it must be converted from ANSI to OEM encoding prior to writing it to a .bat file.
Currently there's no way of getting the current OEM encoding in Tcl.
I suggest extending the "encoding" command to accept the "oem" parameter, that will return the current OEM encoding (just as the "system" parameter returns the current ANSI encoding).
I have no knowledge of OEM encoding on non-Windows systems, so I leave it to the experts (my first thought was it would just return the system encoding).
Probably easiest to make a windows-specific "oem" encoding. If such a thing is needed and not covered by one of our existing encodings, that is.