Preheat to 350°F and line an 8-inch square pan with parchment paper, leaving overhang on two sides for easy bar removal.

In a large bowl, stir rolled oats, all-purpose flour, brown sugar, baking soda, and salt until the mixture is fully combined and crumbly.

Add cold, cubed butter to the oat mixture; use a pastry cutter or your fingertips to blend until it resembles coarse, sandy crumbs.

Reserve two-thirds of the oat mixture and firmly press the remaining mixture into an even layer in the prepared pan.

Bake for 12–15 minutes until the edges are golden brown and the surface looks set; then remove from the oven.

In a saucepan over medium heat, combine cranberries, sugar, water, orange zest, and cornstarch; simmer 5–7 minutes until the mixture is thick and jammy.

Remove the filling from heat and let it rest in the pan for about 5 minutes, stirring gently so it thickens evenly.

Spread the warm cranberry filling evenly over the baked crust, then sprinkle the reserved oat mixture in an even layer on top.

Return to the oven and bake at 350°F for 18–20 minutes until the topping is golden and crisp.

Allow the bars to cool in the pan on a wire rack for at least 1 hour to set fully before slicing into bars.