  The formation of the UV OH spectral lines has been investigated for a range
of stellar parameters in the light of 3D hydrodynamical model atmospheres. The
low atmospheric temperatures encountered at low metallicities compared with the
radiative equilibrium values enforced in classical 1D hydrostatic model
atmospheres have a profound impact on the OH line strengths. As a consequence,
the derived O abundances using 3D models are found to be systematically lower
by more than 0.6 dex at [Fe/H]=-3.0 compared with previous 1D analyses, casting
doubts on the recent claims for a monotonic increase in [O/Fe] towards lower
metallicities. In fact, taken at face value the resulting 3D LTE trend is in
rough agreement with the conventional [O/Fe] plateau. Caution must, however, be
exercised in view of the remaining assumptions in the 3D calculations. We have
verified that the stellar parameters remain essentially unchanged with 3D model
atmospheres provided that the infrared flux method, Hipparcos parallaxes and
FeII lines are utilised, leaving the 3D O abundances from OH lines largely
intact. Greater concern stems from possible departures from LTE in both the
line formation and the molecular equilibrium, which, if present, would increase
the derived O abundances again. Non-LTE line formation calculations with 1D
model atmospheres suggest no significant steepening of the [O/Fe] trend even if
the abundance corrections amount to about 0.2 dex for all investigated stellar
parameters. We note, however, that the 3D case may not necessarily be as
metallicity-independent. The apparent lack of laboratory or theoretical rate
coefficients at the relevant temperatures for the involved molecular reactions
unfortunately prevents a quantitative discussion on the possible effects of
non-equilibrium chemistry.
