Preheat the oven to 350 degrees F. and line a large baking sheet with parchment paper.
In a small bowl, whisk together the 1 ½ cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, ½ teaspoon salt, and set aside.
In the large bowl of a stand mixer, mix together the 1 cup butter and ¾ cups brown sugar and ½ cup granulated sugar until the mixture is light and fluffy.
Add in the 2 large eggs and 1 teaspoon vanilla extract and continue mixing until combined.
Slowly add the flour mixture to the batter until fully combined and then add in the 3 cups old fashioned oats.
Using a large cookie scoop, scoop out the cookie dough and add to the prepared baking sheet.
Bake the cookies for 12 to 15 minutes or until they are golden brown.
Allow the cookies to rest on the baking sheet for a few minutes before removing and placing on the cooling rack.
To make the filling, in a medium bowl using a mixer, beat the ¾ cups butter until smooth and slowly add in the 2 ½ cups powdered sugar until combined. Add in the 1 teaspoon vanilla extract and 1 tablespoon milk and mix thoroughly.
Once the cookies have cooled completely, pipe in the cream filling on the flat side of one cookie and place another cookie on top to form the oatmeal cream pie.