Oxygen probes used for long-term in situ monitoring in the vadose zone generally cannot be retrieved for calibration purposes. Therefore, a method is needed for confirming the calibration of permanently installed O2 probes. We developed a novel in situ calibration checking technique for use with permanently installed probes. The technique was tested at a pyritic tailings disposal site where O2 probes were installed below a bentonite–polypropylene composite cover. The technique involved (i) opening a gas sampling tube to allow atmospheric O2 ingress to the probe’s location via barometric pumping and (ii) analyzing the resultant probe data collected during periods of stable, maximum probe response. Probe calibration checking was done six times during a 12-yr period, and results showed that response data were within 5% of the original calibration. This confirmed the long-term reliability of the probes and identified that the probes’ useable life span extended to at least 12 yr.