  The formation of [O I], O I and OH lines in metal-poor stars has been studied
by means of 3D hydrodynamical model atmospheres. For O I detailed 3D non-LTE
calculations have been performed. While the influence of 3D model atmospheres
is minor for [O I] and O I lines, the very low temperatures encountered at low
metallicities have a drastic impact on the OH lines. As a result, the derived O
abundances are found to be systematically overestimated in 1D analyses, casting
doubts on the recent claims for a monotonic increase in [O/Fe] towards lower
metallicities.
