Preheat the oven to 350°F and line a baking sheet with parchment paper.

In a medium bowl, whisk together 1¼ cups all-purpose flour, ½ teaspoon baking soda, ¼ teaspoon salt, and ½ teaspoon ground cinnamon.

In a large bowl, cream together ¾ cup unsalted butter (softened), 1 cup light brown sugar (packed), and ½ cup granulated sugar until light and fluffy. Beat in 1 large egg (room temperature) and 1 teaspoon vanilla extract until well combined.

Gradually add the dry flour mixture to the butter mixture, mixing just until combined. Do not overmix.

Fold in 3 cups old-fashioned rolled oats, ¾ cup chopped walnuts, and ¾ cup semi-sweet chocolate chips until evenly distributed.

Drop rounded tablespoonfuls of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake for 10–12 minutes, or until the edges are golden and the centers are set.

Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.