Preheat the oven: Set oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.
Cream the butter and sugars: In a large bowl, beat butter, brown sugar, and granulated sugar with a hand mixer until light and fluffy, about 2 minutes.
Add eggs and vanilla: Beat in eggs one at a time, then add vanilla extract, mixing until combined.
Mix dry ingredients: In a separate bowl, whisk flour, baking soda, cinnamon, and salt. Gradually add to wet ingredients, mixing until just combined.
Incorporate oats and mix-ins: Stir in oats and optional raisins or chocolate chips with a wooden spoon.
Shape the cookies: Scoop tablespoon-sized dough portions, roll into balls, and place 2 inches apart on baking sheets. Flatten slightly with a spoon.
Bake: Bake for 10-12 minutes until edges are golden and centers are soft. Cool on the baking sheet for 5 minutes, then transfer to a wire rack.