An oxygen sensor is a device used in vehicles to measure the amount of oxygen in the exhaust gases. It helps the engine control unit (ECU) adjust the air-fuel mixture for optimal combustion, improving fuel efficiency and reducing emissions. Typically located in the exhaust system, the oxygen sensor plays a crucial role in maintaining the performance of the vehicle. By providing real-time data, it ensures that the engine runs smoothly and meets environmental standards, contributing to cleaner air and better overall engine health.