Preheat the oven to 350°F (177°C). Grease an 8×4-inch loaf pan.
Make the streusel: Combine the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the cold butter with two forks or a pastry cutter (preferred). Mix until the streusel resembles coarse crumbs. Stir in the chocolate chips. Set aside.
Make the bread batter: Whisk the flour, baking powder, baking soda, salt, cinnamon, nutmeg, and chocolate chips (or nuts) together in a large bowl until thoroughly combined. Set aside. In a medium bowl, whisk the oil, brown sugar, granulated sugar, egg, vanilla, and zucchini together until combined. Pour into the dry ingredients. Gently whisk until combined; do not overmix. Batter will be semi-thick.
Spread the batter into prepared loaf pan. Bake the bread for 20 minutes. Remove from the oven and sprinkle streusel on top. Using the back of a spoon, gently press the streusel down into the top of the loaf. Return bread to the oven and bake for an additional 25-30 minutes or until a toothpick inserted in the center comes out clean.
Remove the bread from the oven and set on a wire rack. Allow to cool completely before slicing.
Cover and store leftover bread at room temperature for up to 3-4 days or in the refrigerator for up to 1 week.
