Preheat the oven to 350°F (175°C). Grease and line a 9x9 inch baking pan with parchment paper.
Combine the all-purpose flour, rolled oats, brown sugar, melted butter, baking powder, and salt in a mixing bowl and mix until crumbly.
Press half of the crumbly mixture into the bottom of the prepared baking pan.
Mix the chopped rhubarb, granulated sugar, cornstarch, and vanilla extract in another bowl until well combined.
Spread the rhubarb mixture evenly over the base in the baking pan.
Crumble the remaining oat mixture over the rhubarb filling.
Bake in the preheated oven for 30 minutes or until the top is golden brown.
Cool in the pan before slicing into bars.