take the cat off...im assuming the o2 sensor your talking about is that one...take the cat to a shop that has air tools and see if they cant get it out....alot of times a air tool can spin fast enough to get a bolt/nut loose even if its stripped...only other thing i can think of is vise grips and patience....use alot of wd40...spray the o2 sensor prior to trying to remove it like 4 times every 5 mins with wd40 that should help alot