In a medium-sized bowl, whisk together the 1½ cups of all-purpose flour, cornstarch, baking powder, baking soda, cinnamon, and salt until combined. Set aside.
In the bowl of a stand mixer, with a paddle attachment, cream together the butter, light brown sugar, and granulated sugar for 1 to 2 minutes on low.
While the mixer is running, add the eggs one at a time, making sure to completely incorporate each egg and the vanilla extract. Stop the mixer and scrape down the sides of the bowl with a spatula.
While the mixer is on low speed, gradually add the flour mixture, allowing the flour to incorporate before adding more until all the flour has been added. Turn off the mixer and scrape down the sides of the mixer once more.
Add the old-fashioned oats and flour to the mixer and mix on low for another 30 to 45 seconds or until all the oats and pecan flour have been incorporated evenly. Stop the mixer.
In a small bowl, add the chopped fresh cranberries and the remaining 1 tablespoon of flour. Mix to coat the cranberries. This will help them stay evenly dispersed in your cookie batter. Add the flour-coated chopped cranberries to the cookie batter and mix them in by hand, with a spatula, just until evenly incorporated. Do not overmix your dough.
Using a 1½ tablespoon (1.5 inches) cookie scoop, scoop out your cookie dough onto a parchment-lined, large rimmed baking tray. You can place your cookie dough balls right next to each other in rows on the tray. You should be able to fit all your cookie dough balls on one tray. Gently use your fingers to press down on your dough balls so that they are a uniform size (about 1 inch tall). This helps the cookies bake more evenly. Cover your tray with plastic wrap and refrigerate your cranberry pecan oatmeal cookie dough balls for 3 hours or up to overnight.
Once your cookie dough has chilled, preheat your oven to 375°F and line a rimmed baking sheet with a piece of parchment paper or a silicone mat. Place your cookies 2 inches apart on your prepared baking sheet and bake for 11 to 12 minutes, or until your cookies are just set in the middle and lightly golden on the edges. Do not overbake your cookies. They will continue to cook and get a darker color as they cool on the tray.
Allow your cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack.