For ease of replacement, use an OEM sensor. That way, you can cut the plug end of the old wire off and get a 6 point box wrench on the old plug. Use LOTS of PBBlaster because the heat tends to freeze it up.
Some suggest running the engine for 2-3 minutes to get it hot, haven't tried that myself. But do clean the hole very well before inserting the new O2 sensor.
Wear glasses, the rust tends to fall in eyes. And use jack stands, not the dumb tire changing jack.

You did already check for vacuum leaks, right? Those tend to cause the same error message as an O2 sensor.


Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

Not being familiar with your car I would nevertheless suggest that it is a piece of cake if you can a) find it, and b) get a wrench on it. Even if it is a virtually impossible location you can probably buy a special O2 sensor socket for $25-30 and still come out ahead apart from any injuries you might suffer.

Generally, replacing an oxygen sensor is like changing a spark plug. Just unplug the harness and use a wrench to remove it. Sometimes these are hard to reach with a wrench because it may be in a tight space. You can buy the oxygen from the dealer ($$$), use an OE ($$), or get a generic one like the Bosch ($). The generic ones generally require that you cut the wires from the old one and crimp those onto the proper wire on the generic one. The OE generally comes with the same harness that is on the one sold by the dealer. As far as I know, the generic ones work just as well as the OE, just a more complicated intstallation.
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1998 Volvo S70