To make the crumble, combined the oats, flour, brown sugar, cinnamon and butter in a bowl, and mix it until it resembles a large crumb. There should be big and small chunks due to the oats. Set this aside.
Preheat the oven to 400° and line a 12 cup muffin tin with paper liners or lightly grease the cups.
In a large bowl, whisk together the oil and sugar until well combined. Now add in the eggs and whisk until the batter is well combined. Add in the yogurt and mix.
Add the flour, baking soda, baking powder, salt and cinnamon to the wet ingredients. Mix until just combined. You don't want to overmix. The batter will be very thick and this is normal.
Fold in the diced rhubarb.
Divide the batter evenly among the 12 muffins cups. Top with the crumble. Bake for 15-18 minutes or until a toothpick comes out clean!
Let the muffins cool for 5 minutes and then transfer to a wire rack to cool completely.