If it's a simple serial circuit (ECU to O2 sensor), the voltage will change as the resistance changes.
In effect, you'll be looking at a "voltage splitter" (which is a parallel circuit) setup between a multimeter (I've been warned to use one with >10 MegaOhm resistance) and its ground and the O2 sensor and its ground.
Basically as the resistance of the O2 sensor rises, the voltage will drop - an inverse relationship.
Finding the top and bottom of the ranges, as well as what's a good response speed will be the keys to getting a meaningful result.
Cheers and good luck! The folks over at turbobricks may have some suggestions
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