O2 sensors get lazy with age. They still read voltage, but the speed they flip from high volts to low volts increases. The slower the sensor operates, the farther out of control your mixture is, and then that gets hard on the catalyst. Any sensor past 80K is suspicious. There's an original sensor in Dear Old Dad's '98 Monte Carlo; yeah it still "works", and it's not throwing any codes...but the scan tool shows 6 or 8 crosscounts when I'd like to see 20+. That sensor is history come spring.
There is ZERO point to using a penetrating oil on an O2 sensor. The sensor is sealed to the exhaust system using a crushable metal gasket--much like spark plugs used to have. The seal is gas-tight, so liquid is certainly NOT going to get to the threads to do any good.
You will have a lovely oil-spot on the pavement when it all drips off, though.
Either you get the oxy-acetylene torch in there, you get a new, fancy inductive heating element in there, or you do it the old-fashioned way. You buy a 6-point Deep Impact socket--and not just any deep impact socket, you need one that's got the six-point wrenching surface
broached all the way to the drive end. Put a half-inch,
long-handled ratchet or a breaker-bar on the socket, cut the wire off the sensor, and work it until it comes out. AFTER you get it a turn loose, you can try the penetrant. Don't expect miracles, by the time you've turned the sensor out enough to expose the threads to penetrating oil, the threads will already be damaged. Then re-tap the threads in the exhaust system 'cause they WILL be damaged.
When it was me, I bought a Wright 4928 socket. I had a Snap-On deep impact socket of the same size (7/8" x 1/2" drive) but the Snappy and most others have shallow broaching that does not clear the extended body of the O2 sensor. Shallow broaching is probably a stronger design, but it doesn't work for O2 sensors. I have a Snap-On O2 sensor socket that works about half the time. Any socket that's split on one side for the sensor wire to pass through is going to be "spring-y", they open-up when real torque is applied and then they slip and wreck the wrenching surface of the sensor, too.
Amazon.com: Wright Tool 4928 7/8" - 1/2" Drive 6-Point Deep Impact Socket: Home Improvement
https://shop.snapon.com/product/Sensor-Tools/Oxygen-Sensor-Socket/SWR2
A piece-of-crap Vermont American 18mm X 1.5 tap will clean-up a few O2 sensor thread holes, but they're brittle, Chinese, and they're plug taps when
you really want a starting tap. If you find a better tap, get it instead of the junky Vermont American.
Amazon.com: Vermont American 21164 18MM-1.50 High Carbon Steel Metric Spark Plug Tap: Home Improvement
Another possibility is the Lisle thread chaser. Much shorter than the tap, sometimes there's not enough room around the O2 sensor boss for a full-size tap.
Amazon.com: Lisle 12230 Oxygen Sensor Thread Chaser: Automotive