I tried to get my O2 off in my 1995 Civic I had, and I couldn't do it myself.
I used tons of PB Buster (penetrating oil) from the makers of SeaFoam, and still... No luck.

I took it in and they said they couldn't get it out, so they replaced my exhaust manifold and the O2 sensor.

Although I am skeptical, I'm not surprised.