There are two types of oxygen sensors.
1. Zirconia type.
2. Galvanic type or Electrochemical type.

Zirconia type Oxygen probe:

The probe contains a sensing element, comprising a thimbleshaped zirconia cell fitted with inner and outer electrodes at its closed end. The inner electrode is exposed to the flue gas entering the open end of the cell; the outer electrode is supplied with air from a pump or regulator and is therefore exposed to a constant partial pressure of oxygen. Since zirconia is an electrolyte that conducts only oxygen ions at temperatures in excess of 600°C, the voltage generated between the electrodes (i.e. the cell output) is a function of the ratio of the oxygen partial pressure on the inner electrode and its temperature. Therefore, any change in the oxygen partial pressure of the flue gas at the exposed electrode produces a change in the cell output voltage as dictated by the Nernst equation.

Galvanic Cell  type Oxygen probe:

The cell is a diffusion-limited metal/air battery, shown diagrammatically in figure. The oxygen in the sample diffuses through the barrier and reaches the cathode. Here it is reduced to hydroxyl ions which, in turn, pass through the electrolyte to oxidise the metal anode.
A current, proportional to the rate of consumption of oxygen, is generated when the cathode/anode circuit is completed, the cell operating in what is virtually a short-circuit condition. Since the rate at which oxygen reaches the cathode is limited by the diffusion barrier, the cell current is a direct function of this rate, this in turn being a direct function of the concentration of oxygen in the sample