Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a medium bowl, prepare the streusel topping by combining rolled oats, flour, brown sugar, cinnamon, and salt. Add melted butter and mix until the mixture resembles coarse crumbs. Set aside.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
In a separate bowl, whisk together flour, baking powder, and salt.
Gradually add the dry ingredients to the creamed mixture, alternating with milk, beginning and ending with the dry ingredients. Mix until just combined.
Pour the batter into the prepared baking pan, spreading it evenly.
Sprinkle the oatmeal streusel topping evenly over the batter.
Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan on a wire rack before serving.