Although I should say, replacing an O2 is about as easy as it gets-- you literally unplug it from the harness, unscrew the old one, screw in the new one, replug, done. Now, you do need an O2-sensor ...
I have a Ford Ka 2003, just under 60,000 miles with the Check Engine light on. Went to garage yesterday for diagnostic and they said that it was an exhaust sensor issue and the light had gone off but ...