Preheat your oven to 350 degrees Fahrenheit.

In a medium mixing bowl, combine the oats, flour, brown sugar, salt, baking soda, and softened butter until the mixture forms coarse crumbs.

In a 2-quart saucepan, mix the chopped rhubarb, granulated sugar, cornstarch, and water. Cook over medium-high heat, stirring often, until the mixture simmers and the rhubarb softens and the mixture thickens. This should take about 5-10 minutes. Remove from heat and stir in the vanilla extract. Let it cool for about 10 minutes.

Press all but 2/3 cup of the oat mixture into the bottom of a greased or parchment-lined 8x8 inch baking dish. Make sure it's evenly spread and there are no gaps.

Pour the rhubarb mixture over the oat base, spreading it evenly. Sprinkle the reserved oat mixture on top.

Bake in the preheated oven for 30-40 minutes, until the edges are golden and the topping is set.

Let the bars cool completely before cutting and serving. Enjoy!