I bought a replacement O2 sensor from Amazon, the cheapest price around for OEM sensor. Part number: 9202719

Your mechanic is correct, the rear O2 sensor does not have to be replaced in tandom with the front. Just be prepared to replace it eventually.

Removal is easier with the NA engine. The plug for both sensors is on top of the tranny housing and is just visible from the engine bay. It helps to have long arms. The rear O2 sensor has a grey cover. The "top" of the plug is pryed upwards, you may have to use a small screwdriver to get it started. Once the top is up about 1/4 inch, the plug is easily pulled apart.

Follow the wire. The new OEM will have tape markings where the wires are attached to the body. I believe that at least one plastic zip tie is used, just cut it and remember to tie it up again when finishing.

Use liberal amounts of PB Blaster or some other penetrating oil on the O2 sensor and let it soak for at least 1 hour. Breaking it loose is the hardest part. Here is an FCP video that explains a lot, just scroll down the page until you see the video box:
https://www.fcpeuro.com/products/volvo-oxygen-sensor-front-850-c70-s70-v70-15097

The video is for a turbo, so you don't have to remove very much stuff. First get a flashlight and shine it down from the brake fluid reservior and get a visual of the sensor plugs.

I jacked my car up on the driver's side and slid headfirst under the car by the firewall. If you have a long breaker bar, hold the exhaust pipe with one hand and pull on the bar. Once you hear it pop loose, it should come out easily.
I just watched the video, he replaced both sensors. The front O2 sensor is not covered by the plate under the muffler.
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My back feels better when I sit in a Volvo seat