In a large mixing bowl, combine 1 1/2 cups old-fashioned rolled oats, 1 1/4 cups all-purpose flour, 1 teaspoon baking soda, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon salt. Stir until well blended.
Add 1/2 cup packed brown sugar and 1/4 cup granulated sugar to the dry ingredients. Mix thoroughly to evenly distribute the sugars.
Transfer the oatmeal cookie mix to an airtight container or use immediately for baking.
To bake cookies using this mix, preheat oven to 350°F (175°C).
In a separate bowl, beat 1/2 cup (1 stick) softened butter, 1 large egg, and 1 teaspoon vanilla extract until creamy.
Gradually add 1 1/2 cups of the oatmeal cookie mix to the wet ingredients, stirring until combined.
Drop rounded tablespoons of dough onto an ungreased baking sheet about 2 inches apart.
Bake for 8 to 10 minutes or until edges are golden brown.
Remove from oven and allow cookies to cool on the baking sheet for 2 minutes, then transfer to a wire rack to cool completely.