Preheat oven to 350 degrees F.
Adjust the oven racks to the low and middle positions as you will use two trays to bake the cookies. Line 2 large baking sheets with parchment paper and set aside.
In a medium bowl, whisk together flour, baking powder and salt, set aside.
In the bowl of an electric mixer fitted with the wire attachment (or in a large bowl, using a hand mixer) beat the butter on medium speed until creamy. Add the white and brown sugar and beat until fluffy, about 2-3 minutes. Beat in the eggs, 1 at a time.
Using a spatula or a wooden spoon, manually stir in the dry ingredients into the butter-sugar mixture. Once combined, mix in the oats, cranberries and pistachios.
Using an ice cream scoop, scoop dough balls onto the parchment paper covered baking sheets. If you don't have an ice cream scoop, take 2 tablespoons of dough per cookie and roll the dough into a ball. Place the balls 2 inches apart from each other.
Bake until the cookie edges turn slightly golden brown, 22 to 25 minutes. Mid baking, rotate the baking sheets and also move the top one to the bottom and the bottom one to the top to ensure uniform baking.
Remove from oven and let the cookies cool on the baking sheets for 5 minutes before transferring to a cooling wire rack.