Preheat oven: Preheat to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease each cup.
Soak the oats: In a large bowl, combine rolled oats and buttermilk. Let sit for 10 minutes to soften the oats and create a creamy texture.
Prepare the wet ingredients: In another bowl, whisk together brown sugar, granulated sugar, vegetable oil, eggs, and vanilla extract until smooth and well combined.
Combine wet and oat mixtures: Stir the soaked oats into the sugar-egg mixture until blended.
Mix the dry ingredients: In a separate bowl, combine baking powder, baking soda, salt, and cinnamon. Gradually stir the dry ingredients into the wet mixture until just incorporated — do not overmix.
Fold in rhubarb and nuts: Gently fold in the chopped rhubarb and walnuts (if using). The batter should be thick but spoonable.
Fill the muffin tin: Divide the batter evenly among the muffin cups, filling each about two-thirds full.
Bake: Bake for 20–25 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
Cool: Let muffins cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
