Oleo oil is a yellowish fatty fraction extracted from beef tallow, historically important in margarine manufacture. Produced by rendering beef fat and then pressing or centrifuging it to separate the softer olein fraction from the harder stearin, it provided the pliable, spreadable base that early margarines needed before vegetable oil hydrogenation took over. The name survives in “oleomargarine,” the original term for margarine. Beyond food, oleo oil found use in soap making and lubricants. Its decline mirrors the broader shift from animal to vegetable fats in processed foods, driven by cost, supply, vegetarian demand, and concerns about saturated fat and cholesterol. Modern margarines are built on palm, rapeseed, and sunflower oils, but oleo oil remains a textbook example of how fractionation tailors natural fats to functional needs.