So nobody else has this problem: First off using an O2 sensor socket is the best idea instead of a 12 pt or box end of a wrench. That's of minor importance compared to the loosening trick. Heat will loosen things up for you. Wear leather gloves and a long sleeve shirt, and run the engine to loosen the stubborn O2 sensor. Once it's hot, get back under there and it will break loose- be careful not to get burned! I've had to use this trick on many a repair after a DIY job.
I hope that helps someone in the future. Sorry I didn't see this sooner.