Preheat oven to 350°F. Line a baking sheet with parchment paper.
Cream together the softened butter, brown sugar, and white sugar until light and fluffy. Beat in the egg and vanilla extract.
In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the rolled oats and dark chocolate chunks.
Drop rounded tablespoons of dough onto the prepared baking sheet, leaving space between each cookie.
Bake for 10-12 minutes, or until the edges are golden brown and the centers are still slightly soft. The cookies will firm up as they cool.
Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.