Q: O2 sensor not coming off

I will assume you are trying to remove the sensor yourself. The location of the oxygen sensor beneath the car allows rust to take over much of the exposed metal binding some parts together. It becomes very difficult to remove fasteners. Have an experienced technician, such as one from YourMechanic, expose the exhaust pipe to a high heat source like a torch. The exhaust pipe metal will expand and allow some movement from the fastener. If this method fails, the exhaust section may have to be replaced.