The only "law" for Montana state parks that I find is a F&W rule that states you cannot discharge a firearm in a park from April 1 to hunting season. 12.8.202
About city parks etc? Montana has some funny stuff with local governments being able to (but don't have to) restrict unlicensed open carry in a park.
Just because a local government can restrict some OC, does not mean that local government has restricted OC in their parks. Another funny rule is you cannot CC where they serve alcohol, but you can OC where they serve alcohol.
To be safe, you need to check the local town laws if you do not plan on getting a license.