Adjust oven rack to middle position and preheat oven to 350 degrees. Line large baking sheets with parchment paper.

Whisk flour, baking powder, baking soda, and salt in medium bowl and set aside.

In a stand mixer or With a hand mixer, beat butter and sugars at medium-low speed until just combined, about 20 seconds. Increase speed to medium and beat until light and fluffy, 1 to 2 minutes more.

Scrape down sides of bowl and add the egg and vanilla. Beat on medium-low until fully incorporated, about 30 seconds.

Scrape down the mixing bowl again and with the mixer running at low speed, add flour mixture and mix until just incorporated and smooth (don't over mix).

Gradually add oats and mix on low until well incorporated but don't over mix. Give dough final stir with rubber spatula to ensure that no flour pockets remain and oats are evenly distributed.

Using your hand or a 2 Tbsp cookie scoop divide/scoop the dough into around 24 cookies. Roll each cookie into a ball using your hands. Place cookies on prepared baking sheets, spacing them about 2 1/2 inches apart - you can only fit about 8 or 9 per sheet as they will spread. Using fingertips, gently press each dough ball to 3/4-inch thickness (this helps them spread more to develop the tender crisp texture- skip this step if you want and they are still good!)

Bake 1 sheet at a time until cookies are deep golden brown with crisp edges and slightly soft centers, about 13 to 16 minutes.

Transfer baking sheet to wire rack; cool completely on cookie sheet for perfectly crisp cookies (I usually transfer them to a cooling rack to free up my pan and they are great!)