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A study of the O/Fe ratio in metal-poor main sequence and subgiant stars ispresented using the [OI] 6300A line, the OI 7774A triplet, and a selection ofweak FeII lines observed on high-resolution spectra acquired with the VLT UVESspectrograph. The [OI] line is detected in the spectra of 18 stars with -0.5 <[Fe/H] < -2.4, and the triplet is observed for 15 stars with [Fe/H] rangingfrom -1.0 to -2.7. The abundance analysis was made first using standard modelatmospheres taking into account non-LTE effects on the triplet: the [OI] lineand the triplet give consistent results with [O/Fe] increasing quasi-linearlywith decreasing [Fe/H] reaching [O/Fe] ~ +0.7 at [Fe/H] = -2.5. Whenhydrodynamical model atmospheres representing stellar granulation in dwarf andsubgiant stars replace standard models, the [O/Fe] from the [OI] and FeII linesis decreased by an amount which increases with decreasing [Fe/H]. The [O/Fe] vs[Fe/H] relation remains quasi-linear extending to [O/Fe] ~ +0.5 at [Fe/H] =-2.5, but with a tendency of a plateau with [O/Fe] ~ +0.3 for -2.0 < [Fe/H] <-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] ~ -1.0. Use of thehydrodynamical models disturbs the broad agreement between the oxygenabundances from the [OI], OI, and OH lines, but 3D non-LTE effects may serve toerase these differences.Comment: ps file, 18 pages (including 10 figures) - Accepted for publication  in A&

O/Fe in metal-poor main sequence and subgiant stars