  Context. Although oxygen is an important tracer of Galactic chemical
evolution, measurements of its abundance in the atmospheres of the oldest
Galactic stars are still scarce and rather imprecise. At the lowest end of the
metallicity scale, oxygen can only be measured in giant stars and in most of
cases such measurements rely on a single forbidden [O I] 630 nm line that is
very weak and frequently blended with telluric lines. Although molecular OH
lines located in the ultraviolet and infrared could also be used for the
diagnostics, oxygen abundances obtained from the OH lines and the [O I] 630 nm
line are usually discrepant to a level of ~0.3-0.4 dex. Aims. We study the
influence of convection on the formation of the infrared (IR) OH lines and the
forbidden [O I] 630 nm line in the atmospheres of extremely metal-poor (EMP)
red giant stars. Methods. We used high-resolution and high signal-to-noise
ratio spectra of four EMP red giant stars obtained with the VLT CRIRES
spectrograph. For each EMP star, 4-14 IR OH vibrational-rotational lines
located in the spectral range of 1514-1548 and 1595-1632 nm were used to
determine oxygen abundances by employing standard 1D LTE abundance analysis
methodology. We then corrected the 1D LTE abundances obtained from each
individual OH line for the 3D hydrodynamical effects. Results. We find that the
influence of convection on the formation of [O I] 630 nm line in the
atmospheres of EMP giants studied here is minor, which leads to very small
3D-1D abundance corrections (< -0.01 dex). On the contrary, IR OH lines are
strongly affected by convection and thus the abundance corrections for these
lines are significant, 3D-1D ~ -0.2 ... -0.3 dex. These abundance corrections
do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained using
IR OH lines into better agreement with those determined from the [O I] 630 nm
line.

Solar and Stellar Astrophysics