Preheat the oven to 350°F (177°C). Grease an 8×4-inch loaf pan. See notes for muffins.

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.

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 *just* 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. The reason you are adding it after 20 minutes is to prevent it from sinking to the bottom of the bread as it bakes. Return bread to the oven and bake for an additional 25-30 minutes or until a toothpick inserted in the center comes out clean. If you notice the loaf is browning on top or on the edges too quickly, loosely tent the loaf with aluminum foil. Baking times vary so keep an eye on yours.

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.