Spray a fair bit of Liquid Wrench into the threads of the bung on the exhaust pipe. Let it sit for at least 2 or 3 minutes the first time.

Then whack the exhaust pipe a half-dozen times with a dense rubber mallet to break up the gorp caught in the threads.

Repeat 2 or 3 times, adding a bit more Liquid wrench as you go. Then, thread a box wrench that is a TIGHT fit around the hex head at the end of the sensor, and use a crescent wrench ( or another end wrench) to increase your leverage.

To break what's left of the seal, whap the end of the wrench with the mallet. If the box wrench is tight fitting and fully seated, it should work like a charm....

If not, fuck it all and buy a new O^2 sensor. Since you're OBD1, a GI issue sensor from Kragen or AutoZone might work. That's bound to save some $$.

Good luck.