[Walt Fricke]

I've had O2 sensors seize up on me, too. Messed up the threads on the bung, so I switched to a plugged one I had on the other bank. I now use anti-seize on them, being careful not to get any beyond the threads of the sensor.

If the threads on the bung are buggered beyond reuse, you might consider welding that bung shut, drilling a hole at a better angle (but not downward), and welding in a new bung.