Preheat the oven to 375 degrees F. Line a standard 12-cup muffin tin with 10 liners.
For the streusel: Stir together the oats, flour, brown sugar, baking powder, salt and cinnamon in a medium bowl to combine. Add the cold cubed butter and use your hands or a pastry cutter to cut the butter into the dry ingredients until the mixture resembles a clumpy streusel. Set aside.
For the cupcakes: In a medium bowl, whisk together the oil, melted butter and brown sugar until well combined, 1 minute. Add the eggs one at a time, and whisk until well combined. Whisk in the vanilla.
Add the flour, baking powder and cinnamon. Stir to combine with a spatula or spoon. Add the buttermilk and mix until the batter is smooth (do not overmix).
Use an ice cream scoop to divide the batter evenly between the cavities of the prepared pan. Top each generously with streusel.
Bake until a toothpick inserted into the center comes out clean, 15 to 18 minutes. Transfer to a wire rack to cool completely.
For the glaze: Whisk together the confectioners' sugar, cream and vanilla in a small bowl until smooth -- add more cream if necessary to reach a drizzle-able consistency. Use a spoon to drizzle the glaze over the cooled cupcakes.