  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 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. The formation of the triplet O {\sc i} lines was found to be well
described by the classical two-level-atom scattering model, and the non-LTE
correction is practically determined by the parameters of the line-transition
itself without any significant relevance to the details of the oxygen atomic
model. This simplifies the problem in the sense that the non-LTE abundance
correction 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 useful
analytical formula with tabulated numerical coefficients. On the other hand,
our calculations lead to the robust conclusion that LTE is totally valid for
the forbidden [O {\sc i}] lines. An extensive reanalysis of published
equivalent-width data of O {\sc i} 7771--5 and [O {\sc i}] 6300/6363 taken from
various literature resulted in the conclusion that, while a reasonable
consistency 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 abundance
discrepancy was confirmed between O {\sc i} and [O {\sc i}] lines in
conspicuously 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 larger
by $\sim 0.3$ dex at $-3 \la [{\rm Fe}/{\rm H}] \la -2$.
