In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. This should take about 2-3 minutes with an electric mixer. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt. Gradually add the dry ingredients to the butter mixture, mixing until just combined. Fold in the oats and any optional add-ins like raisins or chocolate chips.

Cover the dough and refrigerate for at least 30 minutes. This crucial step helps the cookies maintain their thickness and prevents them from spreading too much during baking.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop 2-tablespoon portions of dough and roll into balls. Place them 2 inches apart on the prepared baking sheets. Bake for 10-12 minutes, or until the edges are golden but the centers still look slightly underdone.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. This resting period helps them set up perfectly.