Preheat the oven to 375°F (190°C). Grease and line a 9x13-inch baking pan with parchment paper or cooking spray.

In a mixing bowl, combine flour, rolled oats, brown sugar, and cinnamon. Cut cold butter into the mixture using a pastry cutter or fork until it forms a crumbly texture. Set aside.

In a large bowl, cream together the butter and sugar using an electric mixer or whisk until light and fluffy. Add eggs, one at a time, mixing well after each addition. Stir in the sour cream and vanilla extract.

In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.

Pour the cake batter into the prepared baking pan, spreading it evenly. Sprinkle the oatmeal streusel topping generously over the batter.

Bake for 25-35 minutes, or until a toothpick inserted into the center of the cake comes out clean. If the streusel begins to brown too quickly, cover the cake loosely with aluminum foil.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. Slice into 12 pieces and serve.