Preheat your oven to 375°F and line a muffin tin with paper liners or grease it lightly.
In a large bowl, combine the flour, oats, brown sugar, baking powder, baking soda, salt, and cinnamon. Stir until well mixed.
In another bowl, whisk together the melted butter, eggs, buttermilk, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.
Gently fold in the chopped rhubarb until evenly distributed throughout the batter.
Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.