18 to 15 is a near 17% drop so hardly a small drop. And yes, the O2 sensor could definitely cause it, but only the upstream. try running a tank of fuel with the sensor unplugged. If the mileage goes back up, replace the sensor. CEL will come on with sensor unplugged but it's nothing to worry about. The computer will be forced to run off programmed fuel tables.I unplugged my upstream sensor when the heater went bad and I was waiting for a replacement NTK sensor from rockauto. I didn't suffer any degradation in MPG.