Oil filled radiators contain a special type of heat transfer fluid. Similarly to how a kettle heats water, there is an element in this fluid that heats it, and this heat is then stored within the radiator while it heats your room.
Electric radiators work just like the radiators in a traditional central heating system, but they’re contained to one unit rather than connected together throughout the whole house. They are filled with a thermodynamic fluid that stores heat to be used to heat your room.