Preheat oven and prepare baking sheet: Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
Cream butter and sugar: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in egg and vanilla: Beat in the egg and vanilla extract until well combined.
Dry ingredients: In a separate bowl, whisk together the oats, baking soda, salt, and cinnamon (if using).
Combine wet and dry: Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Scoop and bake: Drop by rounded tablespoons onto the prepared baking sheet, leaving about 2 inches between each cookie.
Bake: Bake for 12-14 minutes, or until the edges of the cookies are lightly golden brown.
Cool: Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.