Originally Posted by TimFreeh

When operating in "closed-loop" mode the sensor is not going to "sit" at any one voltage for more than a few milliseconds, remember that the sensor is feeding info back to the ECU to adjust the mixture. As the ECU changes the mixture the voltage on the O2 sensor will swing from lean->rich and then back to lean. At idle this pattern repeats itself 2-4 times a second if you have a healthy sensor, maybe once a second or less with a "lazy" or sensor.

If you use a multimeter to watch the O2 sensor voltage you should see it oscillating from .1 to .9 volts in a seemingly random pattern. A scope gives a much more meaning full view of O2 sensor operation.