If you're cheap like me, there's a tab that you can break off the electrical connector so that you can get a closed end wrench over it and down onto the O2 sensor. IIRC, the open end side of a wrench is pretty much useless due to space. The tab that you break off is not the latch itself but instead a latch protector. Its there to protect the latch from accidentally being depressed and releasing the connector. But in this case, I really don't think its necessary.
From what I remember, its a "U" shaped piece of plactic that comes up and around the latch. Just break it off with a pair of pliers or needle nose and the appropriate open end wrench should just barely fit over.
Once you get it on there it might be pretty tight. I think I had to hit the wrench with a rubber mallet to get it loose.
If you're replacing the O2 that's on there now and don't plan to re-use it, you can try and soak the threads with "PB Blaster". Its probably the best comercially available thread loosener out there.
