Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper or silicone baking mats. Set out butter and eggs to reach room temperature if not already done.
Beat softened butter, brown sugar, and granulated sugar on medium speed for 3–4 minutes until light, fluffy, and pale. Scrape down the sides of the bowl at least once during mixing.
Add eggs one at a time, beating well after each addition, then mix in the vanilla extract until smooth. Scrape down the bowl and mix briefly to ensure everything is evenly combined.
In a separate bowl, whisk together flour, baking soda, cinnamon, salt, and nutmeg for at least 30 seconds to evenly distribute. Break up any lumps in the flour.
With the mixer on low, gradually add the dry ingredients to the butter mixture and mix just until no streaks of flour remain — do not overmix or cookies will be tough.
Remove the bowl from the mixer and fold in the oats and raisins by hand using a sturdy spatula or wooden spoon, about 20–30 folds, until evenly distributed throughout the thick dough.
Using a 2-tablespoon cookie scoop, portion dough onto the prepared baking sheets, spacing cookies about 2 inches (5cm) apart — approximately 12 cookies per sheet.
Bake one sheet at a time in the center of the oven for 10–12 minutes, until edges are golden brown but centers still look slightly underdone. Rotate the pan halfway through for even baking.
Let cookies cool on the baking sheet for 5 minutes to set, then transfer to a wire rack to cool completely. Cookies will firm up but remain chewy in the center as they cool.