Preheat oven to 375°F. Line two baking sheets with parchment paper; set aside.
In saucepan, melt butter over medium heat; remove from heat. Stir in cinnamon, then oil; scrape into large bowl. Whisk in brown sugar, granulated sugar, egg, egg yolk, then vanilla.
In separate bowl, whisk together oats, flour, salt and baking soda; stir into butter mixture. Let dough stand 10 minutes.
Using a 3 tbsp scoop, drop dough on prepared pan. Press to flatten into 3-in (8-cm) circles, and bake until edges are golden, 8 to 10 minutes. Let cool on pan for 5 minutes to crisp up before transferring to cooling rack.
Notes
Tip: Reduce oats to 1½ cups and stir in 1 cup semi-sweet chocolate chips to make Chocolate Chip Oatmeal Cookies.
Keyword cookies and bars, easy cookie recipe, Oatmeal Cookies, old-fashioned oatmeal cookies