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, all-purpose flour, brown sugar, baking soda, salt, and ground cinnamon. Mix well.
Pour in the melted butter and honey, then stir until the mixture is crumbly and well combined. Reserve 1 cup of this mixture for the topping.
In another bowl, combine the chopped rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice. Mix until the rhubarb is evenly coated.
Press the remaining 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 in the pan for at least 15 minutes before cutting into squares. For best results, refrigerate for an hour before serving to help them set.