An oxygen sensor measures the proportion of oxygen in exhaust gases to help maintain the optimal air-fuel ratio for emissions control. It works with a three-way catalytic converter to reduce engine emissions. There are several types of oxygen sensors, including zirconia, titanium, and wide-band air-fuel ratio sensors. They function by changing voltage or resistance based on whether the air-fuel mixture is rich, lean, or stoichiometric. Maintaining proper oxygen sensor functioning is important for emissions control and engine performance.