Preheat the oven to 350°F. Cover a cookie sheet with parchment paper.

In a large bowl, blend the butter and sugar with a tablespoon until the mixture is smooth and soft; mix in the egg. In a smaller bowl, combine the all-purpose flour, oat flakes, baking soda and salt. Gradually add the dry ingredients to the butter/sugar mixture, mixing completely after each addition. Incorporate the white chocolate and chopped pistachios into the dough; refrigerate it for at least 30 minutes.

When the dough has cooled, remove it from the refrigerator. Scoop out a third of a cup of dough; quickly roll into a ball and place it on a lined baking sheet. Slightly flatten the dough. Repeat with the remaining dough, spacing them about 2 inches apart.

Bake in a 350°F for 10 minutes, until the cookies are golden and soft to the touch. Transfer the baked cookies to a cooling rack.