If you use an analog multimeter, it should have at least a 20,000 ohms per volt rating. Inexpensive multimeters often are much lower than this. The digital mutimeters have a high ohms per volt rating but sometimes, if the meter's sampling rate is too slow, the meter misses the switching from .1 to .9v.

As you can see, the voltages are pretty low. You should use a range suitable, say 0 to 1.5v.

Also be sure the O2 sensor is up to operating temperature. They don't work when cold.