You can even do without the OpenDocument argument.
So the code looks like this:
// Run the Script
fpsystem ('osascript "/Documenten/Lazarus/Ledenadministratie OSX/db_scriptmsw.applescript"');
And your script need to look something like this:
tell application "Microsoft Word"
Open "/Users/SomeUser/Documents/Some_Document.docx"
Delay (3)
...
...
end tell
I am using the delay to make sure that MS Word has started fully