  Although oxygen is an important tracer of the early Galactic evolution, its
abundance trends with metallicity are still relatively poorly known at [Fe/H] <
-2.5. This is in part due to a lack of reliable oxygen abundance indicators in
the metal-poor stars, in part due to shortcomings in 1D LTE abundance analyses.
In this study we determined the oxygen abundance in the metal-poor halo giant
HD 122563 using a 3D hydrodynamical CO5BOLD model atmosphere. Our main goal was
to understand whether a 3D LTE analysis may help to improve the reliability of
oxygen abundances determined from OH UV lines in comparison to those obtained
using standard 1D LTE methodology. The oxygen abundance in HD 122563 was
determined using 71 OH UV lines located in the wavelength range between 308-330
nm. The analysis was done using a high-resolution VLT UVES spectrum with a 1D
LTE spectral line synthesis performed using the SYNTHE package and classical
ATLAS9 model atmosphere. Subsequently, a 3D hydrodynamical CO5BOLD, and 1D
hydrostatic LHD model atmospheres were used in order to compute 3D-1D abundance
corrections. For this, the microturbulence velocity used with the 1D LHD model
atmosphere was derived from the hydrodynamical CO5BOLD model atmosphere. As in
previous studies, we found trends of the 1D LTE oxygen abundances determined
from OH UV lines with line parameters, such as the line excitation potential
and the line strength. These trends become significantly less pronounced in 3D
LTE. Using OH UV lines we determined a 3D LTE oxygen abundance in HD 122563 of
A(O) = 6.23 +/- 0.13. This is in fair agreement with the oxygen abundance
obtained from OH IR lines, A(O) = 6.39 +/- 0.11, but it is noticeably lower
than that determined using the forbidden [OI] line, A(O) = 6.53 +/- 0.15. While
the exact cause for this discrepancy remains unclear, it is very likely that
non-LTE effects may play a decisive role here.
