In Montana you can't carry a concealed weapon in a place where alcohol is sold, dispensed and consumed. As for OC'ing in a restaurant, it's legal to do but if you're asked to leave, you must abide by the request.
You're not allowed to CC in a bank or other financial institution according to State law. It doesn't say anything about OC'ing in such a place. According to the MT Attorney General's office you can 'theoretically' carry openly in there but the same thing goes, if you're asked to leave by the bank manager, you must do so!