In a bowl, combine the oats and buttermilk; cover and refrigerate for 1 hour.
Meanwhile, grate the zest from the oranges and reserve.
With a small sharp knife, cut off the remaining outer white membranes.
Cut or pull apart the oranges into sections and coarsely chop, taking care not to lose too much of the juice.
Place the chopped sections in a small bowl and add the zest and orange liqueur; set aside at room temperature to macerate as you continue.
While the fruit macerates, make the streusel: combine the brown sugar and flour in a small bowl; work in the cold pieces of butter with your fingers, a fork, or a pastry blender until coarse crumbs are formed.
This can also be done with an electric mixer or food processor, if desired; set aside.
Preheat the oven to 350°; grease eight 4 x 2 1/2 inch mini-loaf pans or spray them with cooking spray.
Add the sugars, oil, and eggs to the cold oat-buttermilk mixture; beat with a whisk until combined.
Stir in the macerated oranges and all the juices.
In a mixing bowl, combine the flour, baking powder, baking soda, and salt; add the wet ingredients and stir with a large spatula just until combined; the batter will be lumpy.
Scrape batter into the prepared pans until just level with the top of the pan.
Sprinkle each loaf with a few tablespoons of the streusel.
Bake in the center of the oven for about 30-35 minutes.
When done, the tops of the loaves will be firm to the touch, the loaves will pull away from the sides of the pans, and a cake tester will come out clean.
Let the loaves stand in the pans for 5 minutes at room temperature before turning them out of the pans onto a rack, then turning them right side up to cool.