I think the O2 light is strictly a reminder that goes off at a preset number of miles--probably an EPA thing. Sounds like what happened is that you had a good running car, the light came on so you tried to do proper maintenance, then someone installed a new sensor and in the process, screwed up the car. Could be:a bad sensor, a bad connection at or near the sensor (always pull apart the connector and see if the pins have moved or gone bad), a wire broke due to age/stress form working in area. Did you put in a factory sensor or an after market? Did it come with the proper connector? You tried to do the right thing. Unfortunately, "no good deed goes unpunished"