Step 1: Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper or silicone baking mats.
Step 2: In a large bowl, using an electric mixer, cream together the softened unsalted butter, light brown sugar, and granulated sugar until light and fluffy (about 2-3 minutes). Scrape down the sides of the bowl as needed.
Step 3: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until just combined.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, salt, and ground nutmeg.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Fold in the old-fashioned rolled oats and raisins with a spatula until evenly distributed.
Step 7: Cover the bowl with plastic wrap and chill the dough for at least 30 minutes, or up to 2 hours. This helps prevent spreading.
Step 8: Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Step 9: Bake for 10-12 minutes, or until the edges are golden brown and the centers are still slightly soft.
Step 10: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.