Rough Idle

My wifes 97 Sable Wagon has a Vulcan 3.0. She ran it low on coolant, and it overheated. Service Engine Soon light came on. The radiator was cracked and that and the Overflow Resevoir were replaced. The engine now does not over heat. There is no oil in water, no water in the oil, and no apparent coolant leaks, so I don't think the head or head gasket is bad. However, the Service Engine Soon light has come back on after the codes were cleared. It initially said there was a misifre on cylinder 1. I have not tuned the car up since I got it, about a year ago. The plugs were toast. Center electrode was burned off down to insulator. Pluga and wires were replaced. Car runs better than the day I got it above idle.
But, it still idles roughly, and the Sevice engine soon light is on. It is steady all the time, but blinks after you idle for a few moments. When you take off, the light returns to steady state. The code now reads bad O2 sensor, but can a bad O2 sensor cause a rough idle. Could it be vacuum? It started when engine overheated. I am confused.
HELP!!!!!! please.