Buttermilk blueberry muffins with streusel topping

Name: Buttermilk blueberry muffins with streusel topping
Prep time: 10 min
Cook time: 25 min
Total time: 35 min
Cuisine: American
Ingredients: 1 ½ cup flour; 3/4 cup sugar; 1/4 teaspoon salt; 2 tsp baking powder; 1/3 cup oil (I use vegetable oil but canola or any neutral oil works); 1 egg; ½ cup buttermilk; 1 ¼ cup blueberries (fresh or frozen); ½ cup flour; 1/4 cup rolled oats; ¼ cup white sugar; ¼ cup brown sugar; 5 tbsp butter (room temperature); Pinch salt