Oxygen-to-iron ratios in Galactic EMP stars plotted versus [Fe / H] ratio. Abundances of EMP giants studied here are marked by triangles: open red triangles – oxygen abundances in the programme stars determined determined in this work with classical 1D model atmospheres; filled red triangles – oxygen abundances corrected for 3D hydrodynamical effects. Solid grey circles show oxygen abundances in red giants, subgiants, and main sequence stars determined using the forbidden [O i] 630 nm line (Gratton & Ortolani 1986; Barbuy & Erdelyi-Mendes 1989; Kraft et al. 1992; Barbuy 1988; Sneden et al. 1991; Fulbright & Kraft 1999; Cayrel et al. 2001; Cowan et al. 2002; Cayrel et al. 2004). Black triangles highlight oxygen abundances determined from [O i] 630 nm line in Cayrel et al. (2004) for the same stars as studied here. The solid line is the evolutionary model of the oxygen abundance from François et al. (2004). We note that [O i] abundances from Cayrel et al. (2004) and the theoretical model from François et al. (2004) were shifted to the reference scale used in our study where solar oxygen abundance is A(O) = 8.66.