In a medium-sized bowl, add the flour, cinnamon, baking soda and salt and whisk well. Set aside.
In a large bowl, add the oil, granulated sugar, brown sugar, eggs and extract and mix with an electric mixer on medium-high speed for about 1 minute.
Add half the dry mixture to the liquid mixture and mix with an electric mixer on medium speed only until just combined.
Add the second half of the dry mixture to the liquid mixture, mixing only until well combined. Scrape down the sides of the bowl and mix again on medium-high speed for only about 10 more seconds.
Add the oats and mix in with a large spoon.
If desired, add in any mix-ins like raisins, cranberries, chocolate chips, nuts etc. Mix them in with a large spoon.
Cover the cookie dough with plastic wrap and chill in the refrigerator for at least 1 hour. (Chilling the dough is necessary because it will make the dough easier to scoop out and will help keep the cookies from spreading out as much when they bake.)
Once the dough is chilled, preheat the oven to 375 degrees F and line two baking sheets with parchment paper.
Use a medium-sized cookie scoop or a tablespoon to add cookie dough mounds to the prepared pans. Leave about 2 inches between each cookie. Do not mash the cookie dough mounds down. They will spread on their own during the baking process.
Bake at 375 degrees F for approximately 12-15 minutes. The cookies will spread out and will be golden brown with a darker brown coloring around the edges of the cookies.
Set the baking sheets onto a cooling rack. The cookies will be delicate at this point, so leave them on the pans to firm up for about 7-10 minutes, then take the cookies off the baking sheet and set them directly onto the cooling rack to cool completely. The cookies will firm up a little once they cool, but will remain chewy.