In a medium bowl, whisk together the rolled oats, flour, baking soda, salt, and cinnamon. Set aside.

In a large bowl or the bowl of a stand mixer, beat the softened butter, brown sugar, and granulated sugar together on medium-high speed for 2-3 minutes until light and fluffy.

Add the egg and vanilla extract to the butter mixture. Beat on medium speed until fully combined, scraping down the sides of the bowl as needed.

Turn the mixer to low speed. Gradually add the dry oat and flour mixture, mixing just until the dry ingredients disappear. Do not overmix.

Using a rubber spatula or wooden spoon, very gently fold in the blueberries. If using frozen berries, toss them in a teaspoon of flour first to prevent bleeding.

Cover the bowl tightly and chill the dough in the refrigerator for at least 30 minutes.

Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

Using a cookie scoop, drop 1.5 to 2 tablespoon-sized balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 12-14 minutes, or until the edges are golden brown but the centers still look slightly soft.

Allow the cookies to cool on the hot baking sheet for 5-10 minutes before carefully transferring them to a wire rack to cool completely.