In a large 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.
Whisk all ingredients together until well mixed and evenly combined.
Transfer the oatmeal cookie mix to an airtight container or jar for storage.
To bake cookies: For every 1 1/2 cups of this mix, add 1/2 cup (1 stick) softened unsalted butter, 1 large egg, and 1 teaspoon vanilla extract. Mix until combined, then drop tablespoon-sized dough portions onto a baking sheet lined with parchment paper.
Bake in a preheated oven at 350°F (175°C) 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.