As the name might suggest, oil filled radiators are filled with a special type of oil that works as a heat transfer fluid. There will be a heating element within this fluid that works similarly to how a kettle heats water. All of this is then held within the metal body, usually made from cast iron.
Oil filled radiators have a distinctive design of curved columns, known as fins, that increase the surface area of your radiator. This allows more air to come into contact with the radiator for optimal heat transfer.
Oil filled radiators use both convection and radiation to heat your space and the people in it.
Convection heating works by heating the air directly in front of the heater. This hot air then rises, pushing cold air downwards to be heated by the heater.
Radiation heat works by directly heating the people and objects in a room, rather than the air. It is a much more efficient method of heating.