  We conducted an investigation on the chemical abundances of 4,316 stars in
the red giant branch (RGB) phase from the recently released APO-K2 catalogue.
Our aim was to characterize the abundance trends of the single elements with
[$\alpha$/Fe], mainly focusing on C, N, and O, which are the most relevant for
the estimation of stellar ages. The chemical analysis of the RGB sample
involved cross-matching data from the APO-K2 catalogue with individual element
abundances from APOGEE DR17. The analysis detected a statistically significant
difference in the [(C+N+O)/Fe] - [$\alpha$/Fe] trend with respect to the simple
$\alpha$-enhancement scenario. This difference remained robust across different
choices for the reference solar mixture and potential zero-point calibrations
of C and N abundances. The primary discrepancy was a steeper increase in [O/Fe]
with [$\alpha$/Fe], reaching a 0.1 dex difference at [$\alpha$/Fe] = 0.3.
Notably, the impact on the evolutionary timescale of such oxygen over-abundance
with respect to the commonly adopted uniform $\alpha$-enhancement is rather
limited. We verified that stellar models computed using an ad hoc O-rich
mixture sped up the evolution by only 1% at [$\alpha$/Fe] = 0.3, due to the
counterbalancing effects of O enrichment on both the evolutionary timescale and
the Z-to-[Fe/H] relationship.
