In a medium saucepan, combine the chopped rhubarb with water, lemon juice, and cornstarch. Cook over medium heat for about 5-7 minutes, stirring frequently until the rhubarb softens and breaks down.

In a large mixing bowl, combine the rolled oats, all-purpose flour, light brown sugar, granulated sugar, baking soda, cinnamon, and salt. Whisk everything together until well mixed.

Add in the softened unsalted butter, using your hands or a pastry cutter to blend until the mixture resembles coarse crumbs.

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan. Press half of the oat mixture firmly into the bottom of the pan.

Spread the cooled rhubarb filling evenly over the oat base. Sprinkle the remaining oat mixture evenly on top.

Place the assembled pan into your preheated oven and bake for 30-35 minutes.

Once baked, remove the pan from the oven and let it cool completely on a wire rack before cutting into squares or bars for serving.