  Varying oxygen abundance could impact the modeling-inferred ages. This work
aims to estimate the ages of dwarfs considering observed oxygen abundance. To
characterize 67,503 LAMOST and 4,006 GALAH FGK-type dwarf stars, we construct a
grid of stellar models which take into account oxygen abundance as an
independent model input. Compared with ages determined with commonly-used
$\alpha$-enhanced models, we find a difference of $\sim$9% on average when the
observed oxygen abundance is considered. The age differences between the two
types of models are correlated to [Fe/H] and [O/$\alpha$], and they are
relatively significant on stars with [Fe/H] $\lesssim$ -0.6 dex. Generally,
varying 0.2 dex in [O/$\alpha$] will alter the age estimates of metal-rich
(-0.2 $<$ [Fe/H] $<$ 0.2) stars by $\sim$10%, and relatively metal-poor (-1 $<$
[Fe/H] $<$ -0.2) stars by $\sim$15%. Of the low-O stars with [Fe/H] $<$ 0.1 dex
and [O/$\alpha$] $\sim$ -0.2 dex, many have fractional age differences of
$\geq$ 10%, and even reach up to 27%. The fractional age difference of high-O
stars with [O/$\alpha$] $\sim$ 0.4 dex reaches up to -33% to -42% at [Fe/H]
$\lesssim$ -0.6 dex. We also analyze the chemical properties of these stars. We
find a decreasing trend of [Fe/H] with age from 7.5-9 Gyr to 5-6.5 Gyr for the
stars from the LAMOST and GALAH. The [O/Fe] of these stars increases with
decreasing age from 7.5-9 Gyr to 3-4 Gyr, indicating that the younger
population is more O-rich.
