soak the o2 in pb blaster for 20 min. ive read in honda tsb's that oil can contaminate the sensor if it is spilled on it. but i think you just need to use some penetrating oil. if that doesnt work take the torch to it and heat it up. im assuming you are junking that o2 so dont worry about fuckin the wires up with the torch. last resort is cut out that section of pipe and replace it with a new peice that has an 02 bung pre-welded in it