1cupfresh blackberrieshalve the berries if they are big
Preheat the oven to 375°F with the rack placed in the center of the oven. Line a muffin pan with cupcake liners and set aside.
In a small bowl, stir together the oats, sugar, flour and cinnamon with a fork. Drop in the butter, and using your fingers, a pastry cutter or two forks, cut the butter into the mixture until you have crumbs. Set the streusel aside.
In a medium bowl, whisk together the flour, sugar, baking powder and salt until combined.
In a small bowl, whisk together the oil, egg, Greek yogurt and vanilla.
Add the yogurt mixture into the flour and use a spatula to mix until just about halfway incorporated.
Drop in the blackberries and mix a little bit more, just until the flour is incorporated – do not over-mix the batter; you want it to be lumpy.
Distribute the batter into the prepared muffin pan and sprinkle with the streusel.
Place in the oven and bake for about 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Remove from oven and place the muffins on a wire rack to cool before enjoying.