In a large bowl, whisk together the flour, pumpkin pie spice, cinnamon, baking soda, and salt. Set aside
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using an electric mixer, cream together the butter, brown sugar, and granulated sugar for 1-2 minutes until well combined. Add the egg yolk and vanilla extract and mix until fully combined. Then mix in the pumpkin puree until well combined.
Slowly mix in the dry ingredients, making sure to stop and scrape down the sides of the bowl as needed. Then mix in the old-fashioned rolled oats until just combined.
Cover the cookie dough tightly with plastic wrap and refrigerate for 30 minutes.
Once the dough is chilled, remove it from the refrigerator. Using a 1.5 tablespoon cookie scoop, scoop the cookie dough and drop onto the prepared baking sheets. Roll the cookie dough into balls and very gently press down with your hand to flatten each ball of cookie dough slightly. Make sure to leave a little room between each ball of cookie dough as they will spread a little while they bake.
Bake in separate batches at 350°F for 9-12 minutes or until the edges of the cookies are lightly browned and the top is set. Remove from the oven and cool on the baking sheet for 5 minutes, then transfer the cookies to a wire rack to finish cooling.