O2 (Oxygen) Sensor - Service Engine Soon Light

What is your question?

The Service Engine Soon light has come on. I was told this was due to my O2 sensor, however the last car mechanic I spoke to said this could be due to bad gas. I have had the light cleared, but it has come back on within a few times of driving. I just hit 40k miles and the car has sat for long periods without being driven (now I am trying to be better about driving it at least 1x week). It is unusual to have to replace the O2 sensor with this low of mileage? I am wondering if something else is the matter.