Step 1: Preheat your oven to 350°F (175°C) and prepare a 9×13-inch baking pan by greasing it or lining it with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large mixing bowl, cream together the softened unsalted butter, packed brown sugar, and granulated sugar. Beat with an electric mixer until the mixture is light and fluffy, which usually takes about 2-3 minutes.
Step 3: Add the large eggs to the creamed mixture, one at a time, beating well after each addition. Stir in the vanilla extract until fully incorporated.
Step 4: In a separate medium bowl, whisk together the dry ingredients: rolled oats, all-purpose flour, baking soda, ground cinnamon, and salt. Gradually add this dry mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.
Step 5: Gently fold in the raisins and the optional chopped walnuts or pecans until they are evenly distributed throughout the cookie bar batter.
Step 6: Transfer the batter into the prepared 9×13-inch baking pan, spreading it evenly with a spatula. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean or with only a few moist crumbs.
Step 7: Allow the baked cookie bars to cool completely in the pan for at least 30 minutes before lifting them out using the parchment paper overhang. Once completely cool, cut them into squares or rectangles and serve.