Oil Filled Radiator Heaters are not cheaper to run than other electric heaters.
The oil filled radiator heaters use the same amount of electricity to operate like the electric fan space heaters do.
An oil filled electric radiator heater uses a heating element that is submerged in oil and that heating element draws 1500 watts of electricity when on high.
That heating element heats the oil up to the hot enough temperature and then shuts off until it needs to heat the oil up again.
So in a way the electric oil filled heater may use slightly less electricity because it's not on all the time unless it's really cold.
But the wattage of the oil filled radiator heater is the same.
Oil Filled Radiator Heaters are safer than electric fan heaters but they can still catch things on fire and you must keep things away from them as well.