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Main Sequence Turn-off stars (MSTO) and subgiant stars are good tracers of galactic populations. We present a study of 41,034 MSTO and subgiant stars from the GALAH survey. Using a grid of stellar models that accounts for the variation of O abundances, we determine their ages with a median age uncertainty of $\sim$9.4 per cent. Our analysis reveals that the ages of high-O stars based on O-enhanced models (OEM models) are smaller than those determined with $\alpha$-enhanced models, resulting in a mean fractional age difference of -5.3 per cent at [O/$\alpha$] = 0.2 and -11.0 per cent at [O/$\alpha$] = 0.4. This age difference significantly impacts the age distribution of thick disc and halo stars, leading to a steeper downward trend in the [Fe/H]-age plane from 8 Gyr to 14 Gyr, indicating a shorter formation time-scale and a faster chemical-enhanced history for these populations. We confirm the V-shape of the normalized age-metallicity distribution $p$($\tau$$\mid$[Fe/H]) of thin disc stars, which is presumably a consequence of the second gas infall. Additionally, we find that the halo stars in our sample can be divided into two sequences, a metal-rich sequence (Splash stars) and a metal-poor sequence (accreted stars), with the Splash stars predominantly older than 9 Gyr and the accreted halo stars older than 10 Gyr. Finally, we observe two distinct sequences in the relations between various chemical abundances and age for disc stars, namely a young sequence with ages $$ $\sim$8 Gyr., Comment: 11 pages, 12 figures

The ratios $r_{01}$ and $r_{10}$ of small to large separations of KIC 2837475 primarily exhibit an increase behavior in the observed frequency range. The calculations indicate that only the models with overshooting parameter $\delta_{\rm ov}$ between approximately 1.2 and 1.6 can reproduce the observed ratios $r_{01}$ and $r_{10}$ of KIC 2837475. The ratios $r_{01}$ and $r_{10}$ of the frequency separations of p-modes with inner turning points that are located in the overshooting region of convective core can exhibit an increase behavior. The frequencies of the modes that can reach the overshooting region decrease with the increase in $\delta_{\rm ov}$. Thus the ratio distributions are more sensitive to $\delta_{\rm ov}$ than to other parameters. The increase behavior of the KIC 2837475 ratios results from a direct effect of the overshooting of convective core. The characteristic of the ratios provides a strict constraint on stellar models. Observational constraints point to a star with $M=1.490\pm0.018$ $M_{\odot}$, $R=1.67\pm0.01$ $R_{\odot}$, age $=2.8\pm0.4$ Gyr, and $1.2\lesssim$ $\delta_{\rm ov}$ $\lesssim1.6$ for KIC 2837475., Comment: Accepted for publication in MNRAS. 9 pages, 10 figures. The ratios $r_{01}$ and $r_{10}$ of KIC 2837475 exhibit an interesting behavior, which can be directly reproduced by stellar models. Adiabatic oscillation frequencies of the models can be downloaded from http://pan.baidu.com/s/1c00LPwk. arXiv admin note: text overlap with arXiv:1508.00955