Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a medium bowl, combine the rolled oats, all-purpose flour, baking soda, ground cinnamon, and salt. Mix well.
In a large bowl, whisk together the melted unsalted butter, brown sugar, and granulated sugar until smooth.
Add the egg and vanilla extract to the butter and sugar mixture, and whisk until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined.
Fold in the raisins and chopped walnuts evenly throughout the dough.
Drop tablespoon-sized portions of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 18 to 20 minutes, or until the edges are golden brown and the centers are set.
Remove from the oven and allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.