Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper.
In a medium bowl, mix together the oat flakes, all-purpose flour, baking soda and salt.
In a large bowl, whisk together the sugars and melted butter until combined. Crack in the egg and pour in the vanilla. Whisk oe more time until the egg is thoroughly incorporated. Add the flour mixture in 2 batches, stirring in between additions until combined. Lastly, fold in the pistachios.
Using a tablespoon OR a cookie scoop, scoop out balls of dough, spacing them about 2 1/2-inches from one another on a the baking sheets. These cookies read a lot, so be sure to leave enough room. Bake for 10 to 12 minutes, or until they spread and the edges are lightly golden brown. Allow to cool on the baking sheet for 5 minutes, then carefully move the cookies onto a cooling rack.