Type II (core-collapse) supernovae occur shortly after star formation and enrich a galaxy with $\alpha$ elements such as O, C, NE, Mg, Ca and Si. On the other hand, Type Ia supernovae occur on a more delayed timescale and produce mostly Fe and heavier elements and negligible amounts of $\alpha$ elements. There is great interest in constraining the [$\alpha$/Fe] ratio of galaxies since that tells you something about the rate at which stars formed in the galaxy as a function of cosmic age.
I have usually seen studies rely on measuring abundances of Mg, Si and Ca relative to Fe, but not oxygen. For reference, the Wikipedia page for the alpha process says
As for oxygen, some authors consider it an alpha element, while others do not. Oxygen is surely an alpha element in low-metallicity Population II stars stars. It is produced in Type II supernovas and its enhancement is well correlated with an enhancement of other alpha process elements.
Why would oxygen not be an $\alpha$ element in high-metallicity environments even though it is produced by Type II supernovae?
Do studies prefer [Mg/Fe], [Ca/Fe] and [Si/Fe] as a proxy for [$\alpha$/Fe] instead of [O/Fe] for an astrophysical reason, or just because the spectral features of Mg, Ca and Si are easier to measure than O?