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Although oxygen is an important tracer of the early Galactic evolution, itsabundance 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 inthe 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 giantHD 122563 using a 3D hydrodynamical CO5BOLD model atmosphere. Our main goal wasto understand whether a 3D LTE analysis may help to improve the reliability ofoxygen abundances determined from OH UV lines in comparison to those obtainedusing standard 1D LTE methodology. The oxygen abundance in HD 122563 wasdetermined using 71 OH UV lines located in the wavelength range between 308-330nm. The analysis was done using a high-resolution VLT UVES spectrum with a 1DLTE spectral line synthesis performed using the SYNTHE package and classicalATLAS9 model atmosphere. Subsequently, a 3D hydrodynamical CO5BOLD, and 1Dhydrostatic LHD model atmospheres were used in order to compute 3D-1D abundancecorrections. For this, the microturbulence velocity used with the 1D LHD modelatmosphere was derived from the hydrodynamical CO5BOLD model atmosphere. As inprevious studies, we found trends of the 1D LTE oxygen abundances determinedfrom OH UV lines with line parameters, such as the line excitation potentialand the line strength. These trends become significantly less pronounced in 3DLTE. Using OH UV lines we determined a 3D LTE oxygen abundance in HD 122563 ofA(O) = 6.23 +/- 0.13. This is in fair agreement with the oxygen abundanceobtained from OH IR lines, A(O) = 6.39 +/- 0.11, but it is noticeably lowerthan that determined using the forbidden [OI] line, A(O) = 6.53 +/- 0.15. Whilethe exact cause for this discrepancy remains unclear, it is very likely thatnon-LTE effects may play a decisive role here.

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. V. Oxygen abundance in the metal-poor giant HD 122563 from OH UV lines