Context : Although oxygen is an important tracer of the early Galactic evolution , its abundance trends with metallicity are still relatively poorly known at \left [ \mathrm { Fe } / \mathrm { H } \right ] \lesssim - 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 where different abundance indicators , such as OH lines located in the UV and IR or the forbidden [ O I ] line at 630 nm , frequently provide inconsistent results .

Aims : In this study we determined the oxygen abundance in the metal-poor halo giant HD 122563 using a 3D hydrodynamical CO ^ { 5 } BOLD 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 .

Methods : 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 CO ^ { 5 } BOLD , 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 CO ^ { 5 } BOLD model atmosphere of HD 122563 .
The obtained abundance corrections were then applied to determine 3D LTE oxygen abundances from each individual OH UV line .

Results : 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 , \chi , and the line equivalent width , W .
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 ) _ { 3 D~ { } LTE } } = 6.23 \pm 0.13 ( \left [ \mathrm { O } / \mathrm { Fe } \right ] = 0.07 \pm 0.13 ) .
This is in fair agreement with the oxygen abundance obtained from OH IR lines , A { ( O ) _ { 3 D~ { } LTE } } = 6.39 \pm 0.11 ( \left [ \mathrm { O } / \mathrm { Fe } \right ] = 0.23 \pm 0.11 ) , but it is noticeably lower than that determined using the forbidden [ O i ] line , A { ( O ) _ { 3 D~ { } LTE } } = 6.53 \pm 0.15 ( \left [ \mathrm { O } / \mathrm { Fe } \right ] = 0.37 \pm 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 .
Oxygen-to-iron ratios determined in HD 122563 using OH UV/IR lines and the forbidden [ O i ] line fall on the lower boundary of the \left [ \mathrm { O } / \mathrm { Fe } \right ] distribution observed in the Galactic field stars at this metallicity and suggest a very mild oxygen overabundance with respect to iron , \left [ \mathrm { O } / \mathrm { Fe } \right ] \lesssim 0.4 .

Conclusions :