What is the postal state abbreviation of Missouri?

To make sure your mail arrives at the correct destination in the United States, you'll need to use the appropriate U.S. state abbreviations. These official state abbreviations have all been standardized by the United States Postal Service (USPS). State postal abbreviations are all two letters, and these two letters are always capitalized without any periods. The postal state abbreviation of Missouri is MO.

Source: Your Dictionary