Re: I've been told it's the O2 sensor????
The O2 sensors detect the air/fuel ratio in the exhaust system. There are two before the catalytic converters and two after. Did they tell you which is bad? If they're sending the wrong code, the computer could be running really lean (very bad) or really rich (also bad, with gas prices these days) and could burn out your catalytic converters prematurely.
They're easy to remove- go to the parts store and get an O2 sensor socket- it's a deep socket with a slit down the side (for the wiring) that slides over the sensor (wiring goes through the slit). Use a regular wrench to get the old one off, unplug it at the sensor, plug in the new one, and put it in just like you removed the old. VERY easy to do. Like I said, they're on your exhaust pipes, located before and after your catalytic converters. Should be the only probes in the piping.