Preheat oven to 350° F. Grease 2 baking sheets or line with silicone baking mats.
Combine butter and sugars in a large mixing bowl. Beat well with an electric mixer until fluffy, about 3 to 5 minutes. Add eggs one at a time and continue beating until well-mixed. Beat in vanilla.
Stir flour, baking soda, cream of tartar, salt & cinnamon in a separate bowl. Add to butter mixture and beat on low speed until mixed. Stir in rolled oats, cranberries and walnuts.
Make 30 balls of dough, using a large spoon or ice cream scoop, and place evenly on prepared baking sheets. Bake for 12 to 15 minutes. Remove from oven and let cool on a wire rack.
HELPING HANDS: Kids can help measure the ingredients.