Begin by heating your oven to 375 degrees F. Line the bottom of an 8x8-inch baking pan with parchment paper. Spray lightly with baking spray. Set aside.
In a bowl, mix together the oats, flour, brown sugar, and salt. Pour the melted butter over this mixture and stir until clumps form. Set aside 1/2 cup of this mixture for the crumb topping. Press the remaining mixture evenly into the bottom of the prepared pan.
In another bowl, mix together the rhubarb, strawberries, sugar, lemon juice, and cornstarch. Spread this mixture evenly on top of the prepared crust. Scatter the reserved flour mixture over the fruit to make a crumb topping.
Bake 30-35 minutes or until the fruit is bubbly and the top "crust" is golden.
Allow the bars to cool completely before slicing and serving.