Step 1: Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper or silicone baking mats.
Step 2: In a medium bowl, whisk together the dry ingredients: all-purpose flour, baking soda, ground cinnamon, ground nutmeg, and salt. Set aside.
Step 3: In a large bowl, cream together the softened unsalted butter, light brown sugar, and granulated sugar with an electric mixer until light and fluffy (about 2-3 minutes).
Step 4: Beat in the large egg and vanilla extract until well combined. Scrape down the sides of the bowl to ensure everything is incorporated.
Step 5: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.
Step 6: Fold in the old-fashioned rolled oats and plump raisins until evenly distributed throughout the dough.
Step 7: Drop rounded tablespoons of dough onto the prepared baking sheets, leaving about 2 inches between each cookie.
Step 8: Bake for 9-11 minutes, or until the edges are set and lightly golden brown, but the centers still look slightly soft. For a chewier cookie, err on the side of underbaking slightly.
Step 9: Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Step 10: Enjoy your warm, chewy oatmeal raisin cookies!