Preheat oven to 350°F (175°C). Line an 8×8 inch baking dish with parchment paper.

In a large bowl, combine oats, flour, brown sugar, baking soda, and salt. Stir in melted butter until the mixture is crumbly and evenly moistened.

Press half of the oat mixture firmly into the bottom of the prepared pan. Set the remaining mixture aside for the topping.

In a medium saucepan, combine chopped rhubarb, sugar, cornstarch, vanilla, and water. Bring to a boil over medium heat, stirring often, until the mixture is thickened and the rhubarb is soft (about 5–7 minutes).

Spread the rhubarb filling evenly over the base in the baking pan. Crumble the reserved oat mixture evenly over the top.

Bake for 35–40 minutes or until the top is golden brown and the filling is bubbling slightly.

Let the bars cool completely before cutting into squares.