An oil heater, also often known as a column heater, is a common form of convection heater used in domestic heating.

Oil heaters consist of metal columns with hollow cavities, inside which oil flows freely around the heater. A heating element at the base of the heater heats up the oil, which then flows around the cavities of the heater by convection. The oil acts as a heat reservoir or buffer, with a relatively high specific heat capacity (approximately 2 kJ.kg-1.K-1) and high boiling point (approximately 150-300 degrees Celsius). The high specific heat capacity allows the oil to store a large amount of thermal energy in a small volume, while the high boiling point allows it to remain in the liquid phase for the purpose of heating, so that the heater does not have to be a high pressure vessel.

Using the oil as a heat reservoir, the heating element heats the oil, which remains warm for a long period of time while the heat is slowly transferred to the room by conduction and radiation. The columns of oil heaters are typically constructed as thin fins, such that the surface area of the metal columns is large relative to the volume of the oil heat reservoir. A large surface area allows more air to be in contact with the heater at any point in time, allowing for the heat to be transferred more rapidly from the heater into the room.

There are several efficiencies that can be measured in regards to heaters: the efficiency of the heater at converting electricity to heat, the efficiency of heating a room with a given amount of power may also be measured. The efficiency of the electrical generator which powers the heater and power loss from transporting the electricity over power lines may be measured also, but this does not change when comparing oil and fan heaters. Measures may also consider how well a heater keeps the temperature of a space above a certain point. Such a measure would find inefficiencies in heating an already warm room. Many heaters (the majority of currently available models) are equipped with a thermostat to prevent this inefficient heating, which in turn reduces running costs compared to heaters without a thermostat. This feature was much more common in oil heaters than in the cheaper fan heaters until recently; thus many older oil heaters will be cheaper and more efficient to run than their contemporary fan heaters that lack the thermostat.