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 mixing bowl, combine the rolled oats, flour, brown sugar, baking soda, salt, and cinnamon. Mix well.
Pour in the melted butter and honey, stirring until the mixture is crumbly and well combined. Reserve about 1 cup of this mixture for topping.
In a separate bowl, combine the chopped rhubarb, granulated sugar, cornstarch, lemon juice, and vanilla extract. Mix until the rhubarb is evenly coated.
Press the oat mixture into the bottom of the prepared baking dish to form a crust. Spread the rhubarb mixture evenly over the crust.
Sprinkle the reserved oat mixture on 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 bubbling.
Allow the bars to cool completely in the pan before cutting into squares. For best results, chill in the refrigerator for at least 1 hour before serving.