Free O₂ ==> 0.0 V

There are two types of oxygen sensors, zirconia (zirconium dioxide) and titania (titanium dioxide).  General Motors automobiles use the zirconia type described herein.

When the zirconia sensor is hot, it responds to the presence or absence of oxygen in the vicinity.  

If there is a dearth of free oxygen, as when unburnt fuel is present in the exhaust, the sensor generates a voltage of somewhat over 0.8 volts.

If there is free oxygen present, the sensor generates zero voltage.

The automobile's computer automatically adjusts the flow of fuel to keep the sensor reading approximately 0.4 volts, thereby keeping the fuel mixture ideal.