Hello "jmolnar," I try to give an answer .... the oxygen sensor, which thou hast made the measurement is installed on a vehicle that meets an old anti-pollution legislation, which allows more emissions, a sensor so slow is not a big problem at least until the idle speed does not fluctuate according to the signal of the sensor (sensor too slow), it is certainly lambda with many miles of work, the important thing is the deltaV or l 'excursion between minimum and maximum, which must be at least 0.7V.
The signal is so "ecstatic", perhaps due to the connection of the black test probe, where you have it connected?
If you connect it to the battery or to the chassis can enter the noise, I saw that also added to the filter to clean the signal.
For a correct measurement should use the negative wire of the oxygen sensor is an electronic mass.
Sorry for my english ....