Preheat oven to 350°F (175°C). Line baking sheets with parchment paper or silicone baking mats.

In a bowl, whisk together flour, baking soda, salt, and cinnamon. Set aside.

In a stand mixer, beat butter, brown sugar, and granulated sugar until light and fluffy, about 2–3 minutes.

Add eggs one at a time, beating well after each. Mix in vanilla extract.

Gradually add dry ingredients to the wet mixture, mixing just until combined.

Fold in oats and raisins (and optional walnuts) with a spatula or on low speed.

Cover and chill the dough for 30–60 minutes for thicker cookies.

Use a cookie scoop to form 1.5-inch dough balls. Place 2 inches apart on the prepared baking sheets.

Bake for 10–12 minutes, until edges are set and cookies are golden brown. Centers should look slightly soft.

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