Although the Oscar-B is a good tank that can be stacked, when you need to create the fuel volume of four to eight of these stacked, it becomes painful to perform a refueling mission and quickly adds to the part count for drop tanks.
If there were a few stock tanks with the Oscar-B radius, but perhaps four times as long (four times the fuel and oxidizer) and another eight times as long (eight times F&O), then you would dramatically reduce the part count and still allow for crafts with side drop tanks that easily fit within the existing cargo bay.