Make the streusel: In a medium bowl combine the oats, flour, dark brown sugar, cinnamon, and salt. Add melted butter and stir to combine. Refrigerate until needed.
Make the batter: Grease a 8x4-inch or 10x3-inch loaf pan with butter or baking spray. Use several pieces of crumpled foil to create an even layer lining the bottom of a 6-quart slow cooker. Adjust the foil as needed to ensure the loaf pan will sit flat inside and the lid will still fit snugly on top.
Combine the flour, baking powder, baking soda, cinnamon, nutmeg, cloves, and salt in a large bowl. In a medium bowl, whisk together the egg, pumpkin, dark brown sugar, and 4 tablespoons melted butter until smooth. Whisk in the buttermilk and vanilla.
Pour the pumpkin mixture into the dry mixture and stir until completely combined. Spread the batter evenly into the prepared loaf pan and tap the pan lightly on the counter to remove any air bubbles. Top the batter with the streusel mixture, pressing down lightly on the streusel to adhere it to batter.
Place the loaf pan in the slow cooker, cover, and cook on low for 4½ to 5 hours or until a toothpick inserted in the center of the cake comes out clean. Keep warm for 1 to 2 hours, if needed. Before serving, let cool for 15 minutes, tap the pan lightly on counter, and invert to remove from the pan. Cut into slices.