Preheat your oven to 350°F (175°C) and line a 9×9 inch baking pan with parchment paper, leaving overhang on the sides for easy removal.
Prepare the cranberry filling by combining cranberries, granulated sugar, orange zest, orange juice, and cornstarch in a medium saucepan. Cook over medium heat for 5-7 minutes, stirring frequently, until cranberries burst and mixture thickens. Remove from heat, stir in vanilla extract, and set aside to cool slightly.
Make the oatmeal mixture by combining oats, flour, brown sugar, baking soda, and salt in a large bowl. Pour in the melted butter and stir until everything is evenly moistened and resembles coarse crumbs.
Press two-thirds of the oat mixture firmly into the bottom of your prepared pan, creating an even base layer.
Spread the cranberry filling evenly over the base layer, leaving a small border (about ¼ inch) around the edges.
Add nuts to the remaining oat mixture if using, then sprinkle this crumble topping evenly over the cranberry layer.
Bake for 25-30 minutes until the top is golden brown and the edges are slightly darker. Look for the cranberry filling to be bubbling slightly at the edges—this indicates it’s done.
Cool completely in the pan on a wire rack for at least 2 hours before lifting out using the parchment overhang and cutting into squares.