Prep the pan and oven: Heat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the paper.

Mix the dry ingredients: In a large bowl, whisk 1 1/2 cups flour, 1 teaspoon baking soda, 1/2 teaspoon salt, and 1 teaspoon cinnamon. Stir in 3 cups rolled oats.

Whisk the wet ingredients: In another bowl, whisk 1 cup packed brown sugar, 1/2 cup granulated sugar, 3/4 cup melted unsalted butter, 2 large eggs, and 2 teaspoons vanilla until smooth.

Combine: Pour wet ingredients into dry. Stir gently until no dry streaks remain. Fold in 1 to 1 1/2 cups of your favorite mix-ins (chocolate chips, raisins, nuts), if using.

Pan and level: Spread the batter evenly in the prepared pan. It will be thick; use a spatula to nudge it into the corners.

Bake: Bake 18–24 minutes, until the edges are set and lightly golden and the center looks just barely done. A toothpick should come out with a few moist crumbs, not wet batter.

Cool and slice: Cool in the pan at least 20–30 minutes. Lift out using the parchment and cut into bars or squares. They firm up as they cool.