The abundance of oxygen was determined for selected very metal-poor G–K stars ( six giants and one turn-off star ) based on the high S/N and high-resolution spectra observed with Keck HIRES in the red through near-IR region comprising the permitted O i lines ( 7771–5 , 8446 ) along with the [ O i ] forbidden line at 6363 \AA .
It turned out that both the abundances from the permitted line features , O i 7771–5 and O i 8446 , agree quite well with each other , while the forbidden line yields somewhat discrepant and divergent abundances with a tendency of being underestimated on the average .
The former ( 7773/8446 ) solution , which we believe to be more reliable , gives a fairly tight [ O/Fe ] vs. [ Fe/H ] relation such that increasing steadily from [ O/Fe ] \sim 0.6 ( at [ Fe/H ] \sim -1.5 ) to [ O/Fe ] \sim 1.0 ( at [ Fe/H ] \sim -3.0 ) , in reasonable consistency with the trend recently reported based on the analysis of the UV OH lines .
We would suspect that some kind of weakening mechanism may occasionally act on the formation of [ O i ] forbidden lines in metal-poor stars .
Therefore , [ O i ] lines may not be so a reliable abundance indicator as has been generally believed .