Oil-fired water heaters typically come with integrated oil tanks that need to be refilled regularly. Heating oil is burned within the unit, creating heat that is applied to the water tank.
Oil-fired water heaters are relatively rare. Those who live in cold areas typically have access to gas lines that provide cheaper heating than oil. While electric water heaters are more expensive to operate than gas-fired water heaters, the savings are not as clear when heating oil is used. Fluctuations in the oil market can make oil-fired water heaters more expensive to operate than electrical units.
Since they come with integrated tanks, oil-fired water heaters are generally more expensive than comparable gas and electrical water heaters. They're also sometimes more expensive due to their niche position in the market. Oil-fired heaters also take up more space, making them difficult to install in rooms designed for smaller units. For these reasons, many people who use heating oil for boilers or furnaces use electrical heaters.
Oil-fired water heaters have a few advantages. Electrical water heaters only function when electrical lines work, and people in remote areas may prefer using heating oil in case electricity is lost. Heating oil is relatively safe to store in large volumes, so people in isolated locations may prefer heating oil to natural gas and propane.