You may have to cut it out / grind it out with a dremal or right angle grinder. The O2 nut may also be worthless. So, in that case, cut off (or grind off) the O2 bung, and weld a new one on.
What type of torch? Just Propane? In that case, try MAPP. An Oxy Acy torch should be able to melt the sensor and bung right off! Heck, I can cut car frames with mine.
Mine were like that, go buy an extraction socket set. Gently hammer on the appropriate sized socket over your 02 sensor (if its junk, just snip the chord at the sensor), and then strong arm it off.
i had to use an oxy cetanline (sp?) torch and we had to get it glowing orange i mean bright orange, then we put th wrench on and busted it off... ur wrench wont fit now so maybe you will cut and drill and tap
Once I heated it up, put the H-pipe (well the sensor) in a vice and unscrewed with the H-pipe (plenty of leverage). That was a last effort though - even the freaking air gun wouldn't do anything with that sucker.
Doesn't heat make things expand? I would think on something threaded, heating it up would make it harder to take out? Of course, science never was my best subject, so I could be totally wrong here.