Let me tell you what I did, PB Blaster when i came home from work let it sit over nite, next day repeat, then the third day after car was driven and warm sprayed and sit for a few more hours, took a 7/8" open wrench or 22mm i believe and you can open it. If you spray it once and expect it to come out, it may be hard, you need to give it some soaking and some capillary action to work. No need to buy special oxygen tool, i did this for the passenger side pre cat...and post cat I had to remove that rubber cylinder with two bolts holding it to get access.

Wow you must be lucky my friend by having your O2 sensors removed easily by hand like that!!! However my (2001 X5) is not rusty neither it's in pretty good shape everywhere...But the fact that I've already used unrelated tool to remove it, so this caused the head sensor now becomes a little loose to have a good torque on it.
I think oxygen sensors are not too hard work to replace, it just the access that seems to be hell to get near by. Right now, I'm using the special oxygen tool device to remove it and this is how I was able to replace the driver front side O2 sensor less than 5mtes. So now I m gonna try your idea like you did for yours and see what's-up.

Big Thanks 4 great foks like you who keeps posting/sharing some personals works done to the forum. And this is how I'm able now to service my (2001 X5 4.4i). So far I've already saved thousands of dollars by doing (oil change, Brake pads/sensors replacement), (spark plug replacement), (Air filter replacement), some electronics devices that were not working like troubleshooting: fuse for (self level susp inact) + cigarette lighter fuse were blown out, I fixed them all by myself!!! Now I'm working on my O2 sensors...