What does a oxygen sensor do?

1980 is when they started putting oxygen sensors in cars and back then they only put one in the car. 1996 is when the OBD II system came out and they started putting more sensors in the cars sometimes up to 4. There is usually one oxygen sensor on the exhaust manifold itself which measures the amount of unburned oxygen. Then there is one after the catalytic converter which measures the oxygen level after its cleaned by the catalytic converter.