Preheat the oven to 425℉ and place a paper liner in each cup of a 12-count muffin tin.
To make the streusel, combine the oats, brown sugar, cinnamon, and flour together in a medium bowl. Add in additional semi-sweet chocolate chips if desired. Then cut in the cold butter with a fork or pastry cutter. Mix until the streusel looks like crumbs and set aside.
To make the muffin batter, whisk together the flour, baking powder, baking soda, salt, cinnamon, nutmeg, and chocoalte chips in a large bowl until combined. Set aside.
In a separate mixing bowl, whisk together the oil, brown sugar, granulated sugar, egg, vanilla, and shredded zucchini.
Pour the dry ingredients into the wet mixture and stir until just combined.
Spoon the batter into each muffin tin, filling each about 3/4 full. Then top each with a generous sprinkling of streusel.
Bake the muffins for 5 minutes at 425℉. Then, reduce the heat to 350℉ and bake for an additional 13-15 minutes or until a toothpick inserted in the middle comes out clean.
Let the muffins rest in the pan for 5 minutes and then transfer to a wire rack to finish cooling.