To a medium mixing bowl, add brown sugar, melted butter, flour, oats, pecans, salt, and cinnamon. Use a fork to mix the crumble ingredients together until combined and set aside.
Preheat your oven to 375 degrees Fahrenheit (191 °C). Line a muffin pan with baking cups, spray the cups with baking spray, and set the pan aside.
To a large mixing bowl, add flour, oats, sugar, baking powder, baking soda, and salt. Whisk ingredients until combined.
To a separate medium mixing bowl add milk, oil, eggs, and vanilla extract. Whisk ingredients together to combine. Pour the egg mixture into the bowl with the flour mixture and use a rubber spatula to combine the ingredients until no more dry flour is visible. Fold in chocolate chips with the rubber spatula.
Using a spoon or cookie scoop, spoon batter into the prepared muffin pan filling cups about ¾ of the way full. Top each muffin cup with about two tablespoons of streusel topping.
Bake muffins in preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with no wet batter. Allow muffins to cool in the pan for 10 minutes then transfer them to a wire rack to cool completely.
Serve muffins room temperature or slightly warm. Enjoy!