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In order to investigate the formation of O {\sc i} 7771--5 and [O {\sc i}]6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of various evolutionarystages. The formation of the triplet O {\sc i} lines was found to be welldescribed by the classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of the line-transitionitself without any significant relevance to the details of the oxygen atomicmodel. This simplifies the problem in the sense that the non-LTE abundancecorrection is essentially determined only by the line-strength ($W_{\lambda}$),if the atmospheric parameters of $T_{\rm eff}$, $\log g$, and $\xi$ are given,without any explicit dependence of the metallicity; thus allowing a usefulanalytical formula with tabulated numerical coefficients. On the other hand,our calculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O {\sc i}] lines. An extensive reanalysis of publishedequivalent-width data of O {\sc i} 7771--5 and [O {\sc i}] 6300/6363 taken fromvarious literature resulted in the conclusion that, while a reasonableconsistency of O {\sc i} and [O {\sc i}] abundances was observed for disk stars($-1 \la [{\rm Fe}/{\rm H}] \la 0$), the existence of a systematic abundancediscrepancy was confirmed between O {\sc i} and [O {\sc i}] lines inconspicuously metal-poor halo stars ($-3 \la [{\rm Fe}/{\rm H}] \la -1$)without being removed by our non-LTE corrections, i.e., the former being largerby $\sim 0.3$ dex at $-3 \la [{\rm Fe}/{\rm H}] \la -2$.Comment: 42 pages including 4 tables and 13 figures, published in A&A, 402,  34

Oxygen line formation in late-F through early-K disk/halo stars