1. Preheat your oven to 350°F (175°C).
2. In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth.
3. Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
4. In another bowl, combine the flour, baking soda, cinnamon, and salt.
5. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
6. Stir in the oats until evenly distributed.
7. Drop rounded tablespoons of dough onto a baking sheet lined with parchment paper.
8. Bake for 10-12 minutes or until the edges are golden brown.
9. Let the cookies cool completely on a wire rack.
10. In a separate bowl, mix the marshmallow fluff and powdered sugar until smooth.
11. Spread a generous amount of the marshmallow mixture on the flat side of one cookie and top with another cookie to form a sandwich.
12. Repeat with the remaining cookies.
13. Enjoy your homemade oatmeal cream pies!

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