Using Electro-galvanic_oxygen_sensor ; Oxygen concentration varying b/w 19% to 30% for ambient air (which should be 21%)
C12 is really bad ju-ju. You must not put much of a capacitor directly on the output of most op-amps or they will oscillate. It would be better to have the RC filter at the input of the op-amp anyway, you could use 10K + 100nF, for example. 

A few hundred pF is okay with this op-amp at G=1 (worst-case) but 100,000pF is not okay. The Zener and the ADC input will have some capacitance, but that's okay. From the datasheet:



As generally good practice, you might want to add a 1M to ground at the input so the reading goes to approximately zero rather than drifting around if the sensor happens to get disconnected.