Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper for easy removal.
In a large bowl, combine the rolled oats, flour, brown sugar, baking soda, salt, and cinnamon. Mix well.
Pour in the melted butter and stir until the mixture is crumbly and well combined. Reserve about 1 cup of this mixture for the topping.
In a separate bowl, mix the chopped rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice until the rhubarb is well coated.
Press the oat mixture evenly into the bottom of the prepared baking dish to form the crust.
Spread the rhubarb mixture evenly over the crust.
Sprinkle the reserved oat mixture over the top of the rhubarb layer.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the rhubarb is bubbly.
Allow the bars to cool in the pan for at least 15 minutes before cutting into squares. For best results, chill in the refrigerator for an hour before serving.