…that will associate a function with the “open file” notification.

One thing I’ve noticed is that if a session (including the “remembered” session) is restored when N++ is starting up, “open file” notifications are not received (for files in the session) by code that is installed via Pythonscript’s startup.py. However, “buffer activated” notifications for these files are received.

Having not tried something like what the OP desires in this scenario, I’m not sure it comes into play, but I thought I’d mention it.

I believe that notepad.menuCommand(MENUCOMMAND.FORMAT_DOS_437) is what you want (DOS CodePage 437 is equivalent to OEM-US). You could bind a script with that (or just record a macro of you selecting Encoding > Character Sets > Western European > OEM-US) and assign to a keystroke of your choice. Or you could write a more complicated PythonScript script that will associate a function with the “open file” notification.