1 cup (2 sticks) unsalted butter, softened 1 cup packed light or dark brown sugar 1⁄2 cup granulated white sugar 2 large eggs 1 tsp vanilla extract 21⁄4 cups all-purpose flour 1 tsp baking soda 1 tsp cinnamon 1⁄2 tsp salt 3 cups rolled oats 1 cup raisins, chocolate chips, or chopped nuts (optional)
Preheat oven to 350°F. Line two baking sheets with parchment paper.
In a large bowl, beat together the butter and both sugars until light and fluffy, about 2-3 minutes. Beat in eggs one at a time, scraping down the sides of the bowl as needed. Stir in the vanilla.
In another bowl, whisk together the flour, baking soda, cinnamon and salt. Slowly mix the dry ingredients into the wet ingredients until just blended.
Scoop rounded tablespoonfuls of dough and roll into balls. Space dough balls 2 inches apart onto prepared baking sheets, lightly flattening each ball.
Bake for 8-10 minutes until lightly browned around the edges but still soft in the center.
Allow cookies to cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.