With a multi meter you are capable of seeing the voltage, with a graphing meter (DSO) you may see a different picture. The frequency, seen with a graphing meter, at which it generates the AC signal displays the sensors ability to react fast enough to the changes in the exhaust stream. The amplitude, which is what you are reading when you use a multi meter, will give you a good idea of the sensors switch points and that it is working, to a point.
Try checking the O2 voltage while driving down the highway at 60mph. If it reads high >500mV constantly the senor sees the exhaust as rich, if it reads low <500mV constantly than it sees the exhaust as lean. An acceptable reading would be 200mV to 800mV sweeping, or constantly changing between these two points.