Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving a 2-inch overhang on at least two of the sides for easy removal. Set aside.
In a large bowl, combine the rolled oats, flour, brown sugar, baking powder, and salt. Add the butter and mix until the dry ingredients are evenly moistened and crumbly.
Transfer 1 cup of the oat mixture to a separate bowl. Stir in the pecans and set aside for the topping.
Press the remaining oat mixture firmly and evenly into the bottom of the prepared pan. Bake for 10 minutes to set the crust.
Meanwhile, make the filling. In a medium bowl, gently toss the raspberries, lemon juice, sugar, flour, and vanilla extract until the berries are evenly coated.
Remove the crust from the oven and spread the raspberry filling evenly over the top. Evenly sprinkle the reserved pecan-oat mixture evenly over the filling.
Bake for 30-35 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Let the bars cool completely in the pan on a wire rack. Once cool, lift them out using the parchment overhang. Slice into 16 squares and serve.