Preheat the oven to 325°F (163°C). Line one or more baking sheets with parchment paper.

In the bowl of a stand mixer fitted with the paddle (or in a large bowl with an electric mixer), add the ¾ cup unsalted butter (room temperature) and 1½ cups brown sugar. Beat on medium speed until the mixture is light and fluffy, about 2 minutes.

Add 1 egg and 2 teaspoons vanilla extract. Beat until fully incorporated, about 1 minute.

Add 1½ cups all-purpose flour, ½ teaspoon baking soda, ½ teaspoon baking powder, and ½ teaspoon salt to the bowl. Mix on low speed just until the dry ingredients are combined into the batter.

Add 2¼ cups old-fashioned oats and mix on low just until evenly distributed. Do not overmix.

The dough will be slightly tacky but should be firm enough to handle. If it is too sticky to roll, add additional all-purpose flour 1 tablespoon at a time, mixing briefly after each addition, until the dough can be rolled by hand.

Roll the dough into balls about 1 to 1½ inches in diameter. Place the balls on the prepared baking sheet, spacing them about 2 inches apart.

Bake in the preheated oven for 10 to 14 minutes, until the edges begin to brown and the centers are set.

Remove the baking sheet from the oven and transfer the cookies to a wire rack. Let the cookies cool completely on the wire rack.