Preheat oven to 425°F. Line a 12-cup muffin tin with paper liners or grease well.
In a small bowl, stir together old-fashioned rolled oats, all-purpose flour, brown sugar, pumpkin pie spice, and salt. Add the melted butter and mix until the mixture forms clumpy, coarse crumbs. Set aside.
In a large bowl, whisk together all-purpose flour, old-fashioned rolled oats, granulated sugar, brown sugar, baking soda, baking powder, salt, and pumpkin pie spice.
In a separate bowl, whisk together canned pumpkin puree, plain kefir, eggs, oil, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir gently just until combined. Don’t overmix, a few lumps are fine. Let the batter sit for 10 minutes
Divide batter evenly among the muffin cups, filling each about three-quarters full. Sprinkle streusel generously over the tops, pressing gently so it adheres.
Bake 7 minutes. Without opening the oven, lower the heat to 350°F and bake an additional 15-20 minutes or until a toothpick inserted in the center comes out with a few crumbs.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.