In a large mixing bowl, combine 2 cups old-fashioned rolled oats, 1 1/4 cups all-purpose flour, 1 teaspoon baking soda, 1/2 teaspoon ground cinnamon, and 1/2 teaspoon salt.
Add 1/2 cup packed light brown sugar and 1/2 cup granulated sugar to the bowl.
Stir all ingredients together until evenly mixed.
Transfer the oatmeal cookie mix to an airtight container or a resealable plastic bag for storage.
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, 1 large egg, and 1 teaspoon vanilla extract until creamy.
Add 1 cup of the oatmeal cookie mix to the wet ingredients and stir until combined.
Drop rounded tablespoons of dough onto a parchment-lined baking sheet, spacing about 2 inches apart.
Bake for 10-12 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.