In a large bowl add butter and sugars and beat at medium speed until fluffy, 2 to 3 minutes, stopping to scrape sides of the bowl.
Add eggs, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
With the mixer on low speed, gradually add flour mixture to butter mixture, beating just until combined, about 1 minute.
Using a rubber spatula, gently fold in oats, chocolate, and walnuts.
Cover and refrigerate for at least 1 hour.
Preheat oven to 180°C (350°F). Line baking sheets with parchment paper.
Scoop dough by heaping tablespoonfuls (about 45 grams), and roll into balls. Place about 5 cm apart on prepared pans.
Bake for 12 to 15 minutes, rotating pans halfway through baking.
Stored in an airtight container at room temperature for up to 5 days.
Let cool on the pan for several minutes then transfer to a wire rack to cool completely.