Prep the Streusel: Combine the streusel ingredients (melted butter, flour, oats, brown sugar, and cinnamon) in a bowl and stir to combine with a fork - this will help keep clumps. Set aside.
Muffins: Preheat the oven to 350°F. Line 12 muffin tins or rub with nonstick oil. Toss the diced pears with 1 teaspoon flour to prevent them from sinking to the bottom of the muffin.
In a large bowl, stir together the all-purpose flour, baking soda, baking powder, salt, cinnamon, granulated sugar, and brown sugar. Set aside.
Whisk together melted butter (it should have cooled slightly as to not cook the eggs), eggs, yogurt, and vanilla. Pour the wet ingredients into the flour mixture and stir until slightly combined. Add the oats and diced pear, stirring until just fully combined. Lumps are OK - you don't want to overmix the batter. Divide the batter evenly among the muffin tins, filling them about 3/4s full.
Evenly divide the oat streusel among the muffins, gently pressing it down on top of the muffins. Bake for 18-20 minutes until muffins bounce back slightly when tapped, and/or when the internal temperature reaches 190°F. Let cool slightly, and serve warm for best results.
The muffins are best eaten the day of, so the topping keeps its crunch. Otherwise store at room temperature uncovered to keep a crisp streusel for the next day.