A DMM won’t react quickly enough to read the voltage cross counts of the upstream O2 sensor.
The reason a scanner is used is because it’s easier to connect to the vehicle to get live data.
With an oscilloscope, you’ll have to connect directly to the signal out wire from the O2 sensor.
But an oscilloscope can be used. Set the scope up to read 0-1.0 VDC at a sample rate of .20 seconds.
Here are samples of what the signal should and shouldn’t look like.
The downstream O2 sensor signal should be flat-lined between .4-.8 VDC.
If there’s any cross counting occurring in the downstream O2 sensor signal, it indicates that catalyst efficiency is falling off.