In order to investigate the formation of O i 7771–5 and [O i] 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. A useful formula with tabulated numerical coefficients is presented, applicable to evaluating NLTE corrections for given Wλ and atmospheric parameters (Teff, log g, ξ). Our calculations suggest a possibility of positive NLTE correction (i.e., NLTE lineweakening) for the [O i] 6300/6363 lines, though its extent is practically insignificant in most cases. Reanalysis of published equivalent-width data of O i 7771–5 and [O i] 6300/6363 taken from various literatures confirmed that, while a reasonable consistency of O i and [O i] abundances was observed for disk stars (−1 � [Fe/H] � 0), the existence of a systematic abundance discrepancy was confirmed between O i and [O i] lines in conspicuously metal-poor halo stars (−3 � [Fe/H] � −1) without being removed by our non-LTE corrections, i.e., the former being larger by ∼ 0.3 dex at −3 � [Fe/H] � −2. Inspecting the parameter-dependence of this discordance, we suspect that [O i] lines may be weakened by some effect possibly dependent upon the surface gravity, rather than attributing the cause of errors to the side of O i lines. This interpretation naturally suggests the preference of the “rising ” tendency to the “flat ” trend as the [O/Fe] ratio of halo stars toward a very metal-poor regime
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