Chop the rhubarb and set it aside. Grease a 9 x 5-inch aluminum loaf pan with butter.
In a large bowl, whisk the flour, baking powder, baking soda, and salt. Make a well in the center.
In a medium bowl, whisk the granulated sugar, buttermilk, melted butter, neutral oil, eggs, and vanilla extract.
Make the streusel: In a small bowl, mix the sugar, all-purpose flour, cinnamon, and optional orange zest. Cut the butter into small pieces, then use your fingers to rub it into the dry ingredients until well distributed and crumbles form.
Pour the wet ingredients from the batter (Step 4) into the well in the dry ingredients. Mix gently until just combined and there are no longer streaks of flour.
Add 1 2/3 cups of the diced rhubarb and fold it in with a spatula, taking care not to overwork the batter.
Pour the batter into the prepared pan. Sprinkle the top with 1/3 cup of the chopped rhubarb, then sprinkle evenly with the streusel.
Place in the oven, adding a baking sheet on the rack underneath. Bake until golden brown and a toothpick inserted in the center comes out with a few crumbs, rotating the pan halfway through baking, about 55 to 65 minutes. Allow to cool in the pan for 30 minutes.
Run a knife around the edge and invert the loaf onto a cooling rack. Allow to cool fully to room temperature (about 1 hour) before slicing into pieces. This bread is best the day it’s made, but you can store wrapped in aluminum foil and refrigerated for up to 10 days: make sure to bring to room temperature before enjoying. Alternatively, store frozen for up to 3 months.