In a medium bowl, whisk oats, baking soda, baking powder together. In a medium metal bowl, beat softened butter and sugars together. Beat in eggs and vanilla. Using a wooden spoon, gradually stir in flour mixture until well combined. Fold in chocolate chips and walnuts. Cover with plastic wrap and refrigerate for 1 to 2 hours. Preheat oven to 180° C (350° F). Scoop dough into a 1/3 cup (75 mL) and drop on to a lightly greased cookie sheet 4 inches (10 cm) apart. Flatten with your hand or a lid from a large jar. Bake for 10 to 15 minutes or until golden brown. Let cool on a wire rack.