In a medium bowl, whisk together the rolled oats, all-purpose flour, baking soda, ground cinnamon, and salt until well combined.
Add the light brown sugar and granulated sugar to the dry mixture and stir thoroughly to combine.
Fold in the raisins and chopped walnuts evenly throughout the mixture.
Layer the mixture into a clean 1-quart (4-cup) glass jar, pressing down gently to compact slightly.
Seal the jar tightly with a lid and store in a cool, dry place until ready to bake.
To bake the cookies, preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a separate bowl, beat together 1/2 cup (1 stick) softened unsalted butter, 1 large egg, and 1 teaspoon vanilla extract until creamy.
Add the cookie jar dry mix to the wet ingredients and stir until combined.
Drop rounded tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10 to 12 minutes, or until the edges are golden brown and centers are set.
Remove from oven and allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.