The principle of the oxygen sensor in the exhaust stream is that of a 
thermistor which is yes, a heater.  A controlled amount of current is 
supplied to the thermistor and carefully measured on a continual basis. 
If the amount of current consumed by the thermistor decreases, then it 
is obvious that the thermistor is being subjected to heat from a source 
other than the supplied current.  In simple terms, the O2 sensor is 
measuring the exhaust gas temperature.  The computer compares the heat 
with a calculated factor based on rpm, throttle position, manifold 
pressure and all the other factors that determine how much fuel to 
burn.  Get the idea?   Lessie .-)