Preheat the oven to 375 degrees F.
Place muffin liners in 2 1/2-inch muffin tins.
Place the brown sugar, pecans, oats, and butter in a food processor or blender and pulse 10 times until mixture is crumbly and well combined. Set aside.
For the batter, place the sugar, oil, eggs and pumpkin in a large mixing bowl and beat with electric mixer set on medium-low until well combined, 1 minute. Add the flour and spices to the bowl and pour in a little of the water. Beat on low until just combined. Continue adding the rest of the water, beating on low speed until well combined. Stir in chopped pecans.
Spoon batter into the muffin pans, filling three-quarters full. Spoon 2 teaspoons of the topping on the top of each muffin. Place muffin pans in the oven and bake 20 – 25 minutes, or until muffins spring back when lightly pressed with your finger and pecans are golden brown.
Remove pans to a rack to cool for a few minutes, then carefully remove muffins to the rack to cool completely. Repeat the process as needed to bake all the muffins.
To freeze, place cooled muffins in zipper-lock bags and store in the freezer for up to six months.