The vadose zone and ground water environments are a sink for atmospheric O2. The pathways and rates of O2 consumption are primarily related to the availability and rate of oxidation of key reductants (e.g., organics, sulfides), through a combination of biological or abiotic reactions. The range in δ18O of O2 in the subsurface is large, from +20‰ to +39‰ (Vienna Standard Mean Ocean Water) in the vadose zone and from +12‰ to +46‰ in ground water. The aggregated O2 isotope fractionation by consumption (αk) was found to range from 0.970 to 1.300 and 0.980 to 1.030 in vadose zones and aquifers, respectively. These data suggest the δ18O patterns in both unsaturated zones and aquifers can be attributed to microbially mediated reactions (αk= range from 0.975 to 1.000), but there are apparently other inverse isotope fractionating processes (αk > 1.000). Circumstantial evidence suggested O2 processed during the sulfide oxidation and precipitation of Fe-oxyhydroxides process (or other unidentified processes) could be the cause of the significant 18O depletions. Overall, δ18O data from vadose zones and ground water revealed that isotope fractionation by consumption of gaseous and dissolved O2 in the subsurface and ground water environments is more complicated than what has classically been attributed solely to geomicrobial respiration. Given the questions and inexplicable data arising from this study, further detailed research on O2 consuming processes in the Earth’s subsurface and ground water is warranted.