I suggest you go here and click on your state and read the laws particular to your state. I only skimmed it so can answer your specific question but this would be a good place to start.
Also the STICKY at the top of the Montana section where you posted says this about OC .....
Montana is an open carry state. That means that most anyone who can legally own a firearm can carry it unconcealed anywhere in Montana with only a few exceptions like schools and federal buildings. Montana expressly permits drawing or presenting a weapon to an aggressor threatening bodily harm. MCA 45-3-111 http://data.opi.mt.gov/bills/mca/45/3/45-3-111.htm
Localities are substantially preempted, but have limited power to prevent and suppress the carrying of concealed or unconcealed weapons to a public assembly, publicly owned building, park under its jurisdiction, or schools. MCA 45-8-351 http://data.opi.mt.gov/bills/mca/45/8/45-8-351.htm
You only need to be 14 years old to open carry in Montana. If a minor under 14 wishes to open carry they must be accompanied by an adult who is the child’s parent or guardian or who has been authorized by the child’s guardian to supervise the child. MCA 45-8-344 http://data.opi.mt.gov/bills/mca/45/8/45-8-344.htm (Federal law still applies)