In a large mixing bowl, combine 2 cups old-fashioned rolled oats, 1 1/2 cups all-purpose flour, 1 cup packed light brown sugar, 1/2 cup granulated sugar, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.
Stir the dry ingredients together until evenly mixed.
Fold in 1/2 cup chopped walnuts.
Transfer the oatmeal cookie mix to an airtight container or a large resealable plastic bag.
Store the mix in a cool, dry place until ready to use.
To bake cookies from the mix: Preheat oven to 350°F (175°C). In a separate bowl, beat 1/2 cup (1 stick) softened unsalted butter, 2 large eggs, and 1 teaspoon vanilla extract until creamy.
Add 2 cups of the oatmeal cookie mix to the wet ingredients and stir until combined.
Drop rounded tablespoons of dough onto a parchment-lined baking sheet about 2 inches apart.
Bake for 10 to 12 minutes or until edges are golden brown.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.