In a large mixing bowl, beat 1 cup softened unsalted butter with 1 cup packed brown sugar and 1/2 cup granulated sugar until light, fluffy, and creamy (about 3-4 minutes).
Add 2 large eggs one at a time, beating well after each addition. Stir in 1 teaspoon vanilla extract until fully combined.
In a separate bowl, whisk together 1 cup all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.
Gradually add the dry ingredients to the wet mixture, mixing on low speed or folding with a spatula just until incorporated. Avoid overmixing.
Gently fold in 3 cups rolled oats and 1 cup raisins.
Preheat oven to 350°F (175°C). Drop rounded balls of dough onto lined baking sheets about 2 inches apart. Bake for 10-12 minutes until edges are golden and centers look set but soft.
Let cookies rest on the baking sheet for 5 minutes before transferring to a cooling rack to cool completely.
