I did a search and did not find a concise guide to removing a badly rusted o2 sensor. I did find the usual suspects in removing rusted items: lots of PB Blaster, apply heat, use closed end wrench, and deep well socket and ratchet.

I recently and successfully removed the oxygen sensor on my 1992 Firebird V6. It was heat and rust welded very firmly. Let me share what worked well for me.

I tried PB Blaster, wrench, torch, and ratcheting which did not work. Finally, I gave myself a dope slap and tried using my $40 (on sale) Harbor Freight electric impact wrench. Incidentally, one of my absolute best tool buying decisions for working on my old 3rd gen.

I unplugged the sensor wire connection and cut the wire off right at the top of the sensor. I used a deep well impact socket and my cheap impact wrench. However, I must have held the trigger on the impact wrench for 3 or more minutes before it finally spun off the sensor. Other than my hands feeling like jelly for a few minutes afterward, it was a pretty slick way to unscrew the old rusted sensor.

I also understand that the sensor size and threads are the same as a spark plug. Thus, if you screw up the threads, a spark plug thread cleaner should allow you to clean up the threads and more easily install the new o2 sensor.