Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan.
Make the streusel: in a small bowl, mix the flour, brown sugar, and cinnamon. Cut in the cold butter with a pastry cutter or your fingers until crumbly. Refrigerate while you make the batter.
In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and cinnamon.
In a separate bowl, whisk together the vegetable oil, sour cream, eggs, and vanilla until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Fold in the diced apples.
Pour the batter into the prepared loaf pan and sprinkle the streusel evenly over the top.
Bake for 55-65 minutes, until a toothpick inserted in the center comes out clean. Cool in the pan for 15 minutes, then remove to a wire rack.