Combine the oats, flour, baking powder, baking soda, salt, and cinnamon in a large bowl. Mix well. In a separate bowl, beat the butter, shortening, and sugars with an electric mixer for 2 minutes. Beat in the eggs, one at a time. Blend in the vanilla. Using a wooden spoon, stir in the dry mixture, about half at a time. Add the chocolate chips and walnuts. Stir until evenly mixed. Cover and refrigerate for 1 hour (or longer; if longer, let sit at room temperature for 10 to 15 minutes before shaping).
Preheat the oven to 350 degrees F. Lightly butter two large, shiny baking sheets or line them with parchment paper.
Using 1/4 cup of dough per cookie, shape the dough into balls and place them on the baking sheet, leaving 2-1/2 inches in between. Flatten the balls into 1/3-inch-thick disks with your fingers. Bake one sheet at a time on the center oven rack for 13 to 14 minutes, turning the baking sheet 180 degrees midway through the baking time. When done, the cookies will have browned very lightly and still look moist beneath the surface. Do not overbake. Cool for 2 to 3 minutes on the baking sheet. Transfer to a rack to cool completely. If you’re using only one baking sheet, let it cool between batches.
Coconut Cowboy Cookies
Add 1 cup of flaked sweetened coconut to the dough along with the chocolate chips.
Bandanna Cowboy Cookies
Omit the chocolate chips and substitute 1 cup of white chocolate chips and 1 cup of Craisins. (The white and red suggest a bandanna.)
Peanut Butter Cowboy Cookies
Omit the chocolate chips and substitute 2 cups of peanut butter chips. Or, use 1 cup each of peanut butter chips and chocolate chips.