I'm aware, ST, but what that means is: either there IS no local ordinance restricting Oklahoma teenage girls from open carrying; or this cop should have cited and disarmed them on the spot.
The girl will have to abide by the Oklahoma law and respect Missouri law, at the same time, including the patchwork of local ordinances. If it were me (from Oklahoma in Missouri), I would just carry concealed - and that would give me, personally, less to be concerned about and know.