O2 sensor off car

It basicly tells the ECU the amount of O2 in the exhaust compare to the amount ambient O2. When the engine is running rich there are very less oxygen particals present in the exhaust and more on the ambient side so there is more pressure as the outside Oxygen particals want to go where there is less oxygen particals so this pressure creats a higher voltage which at max it gets close to 1 Volts to the ECU. When we have a lean mixture there are more oxygen particals present in the exhaust manifold so therefor the pressure between each side of the o2 sensor is less and this causes less voltage output to ECU.
One of the draw backs of these sensor is that they need to be heated up first to around 300 celsious before it starts working . The ECU uses this sensor to go into closed loop as soon as it has been heated up and in working temerature to balance the air/fuel ratio mixture and it constantly going lean /rich .... as it is trying to find the optimum mixture for emmisions and performance .