According to About.com expert Timothy Thiele, oil heaters are electric appliances that warm the surrounding air by passing hot oil through a series of metal tubes. Oil is the ideal filling for space heaters because it heats quickly and gets extremely hot without boiling. As the heater runs, the hot oil heats its metal tubing, which has poor heat retention and quickly releases the heat into the air.
The most effective oil heaters have many flattened, thin metal tubes. Maximizing the tubes' exterior surface area brings more air into contact with the heater, reducing the amount of time necessary to heat the surrounding space.
Oil heaters quickly become too hot to touch and take up to an hour to cool down after use. About.com expert Timothy Thiele writes that some space heaters contain water instead of oil. These cool down more quickly than oil heaters. Some other types of space heaters have visible heating elements that glow red when hot, but oil heaters lack this feature. They also produce heat more slowly than other heaters.
Oil & Water explains that oil space heaters are fully electric and do not release toxic fumes or carbon monoxide, a clear advantage over kerosene heaters. Their concealed heating elements also reduce the risk of fire, a serious danger of exposed heating elements.