Preheat oven: Set oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone mat.
Cream butter and sugars: In a large bowl, beat butter, brown sugar, and granulated sugar until light and fluffy, about 2 minutes.
Add wet ingredients: Mix in egg and vanilla extract until smooth.
Combine dry ingredients: In a separate bowl, whisk flour, baking soda, cinnamon, and salt. Stir into wet ingredients until just combined.
Add oats and mix-ins: Fold in oats and optional raisins or chocolate chips.
Shape cookies: Scoop tablespoon-sized dough balls, place 2 inches apart on the baking sheet, and flatten slightly.
Bake: Bake for 10–12 minutes until edges are golden. Cool on the baking sheet for 5 minutes, then transfer to a wire rack.