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Oxygen is a powerful tracer element of Galactic chemical evolution.Unfortunately, only a few oxygen lines are available in theultraviolet-infrared stellar spectra for the reliable determination of itsabundance. Moreover, oxygen abundances obtained using different spectral linesoften disagree significantly. In this contribution we therefore investigatewhether the inadequate treatment of convection in 1D hydrostatic modelatmospheres used in the abundance determinations may be responsible for thisdisagreement. For this purpose, we used VLT CRIRES spectra of three EMP giants,as well as 3D hydrodynamical CO$^5$BOLD and 1D hydrostatic LHD modelatmospheres, to investigate the role of convection in the formation of infrared(IR) OH lines. Our results show that the presence of convection leads tosignificantly stronger IR OH lines. As a result, the difference in the oxygenabundance determined from IR OH lines with 3D hydrodynamical and classical 1Dhydrostatic model atmospheres may reach -0.2 ... -0.3 dex. In case of the threeEMP giants studied here, we obtain a good agrement between the 3D LTE oxygenabundances determined by us using vibrational-rotational IR OH lines in thespectral range of 1514-1626 nm, and oxygen abundances determined from forbidden[O I] 630 nm line in previous studies.

Oxygen in the early Galaxy: OH lines as tracers of oxygen abundance in extremely metal-poor giant stars