I know that it is legal to OC in Missouri.
I am originally from STL but I am currently stationed in Washington.
I will be in STL at the end of August and I plan on bringing my XD40 along with me.
I understand that Missouri accepts any states CPL.
Is that enough to OC or should I CC????"
You would be better off to CC, especially in the St. Louis area. RSMo. 21.750 pre-empts firearms legislation to the state legislature except for the regulation of open carry.
That means that the state is a minefield of municipalities with regulations against open carry and no way to know which ones unless/until you are arrested for violating such an ordinance. The St. Louis area, especially, has been known for such ordinances; in many cases you may not even know when you pass from a village without such a law into a neighboring village forbidding open carry.
As you said, Missouri does honor any permit to carry concealed issued by any other state or political subdivision and this is good throughout the state. We hoped to get pre-emption passed with regard to open carry as well this year but the NRA seems to have told our legislators it had other priorities such as owning silencers.