Move an oven rack to the center of oven and preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Mix the white sugar, 3 tablespoons of flour, salt, eggs, orange zest, and ginger together in a bowl until well combined; stir in the rhubarb. Pour the rhubarb mixture into the bottom of the prepared baking dish.
Thoroughly combine 1/2 cup flour, brown sugar, butter, and cinnamon by pulsing in a food processor or blender. Stir in the oatmeal; crumble the oatmeal mixture over the rhubarb. Gently pat the topping down to make a crust.
Bake on the center rack of preheated oven until the topping is lightly golden, the rhubarb has fallen apart, and the juices are very thick and bubbling, 40 to 50 minutes. Check frequently after 30 minutes to see if bubbles are thick.
