  This paper presents the results of a detailed theoretical investigation of
the impact of non-LTE effects and of granulation inhomogeneities on the derived
iron and oxygen abundances in the metal-poor halo subgiant HD140283. Our
analysis is based on both the `classical' one-dimensional (1D) stellar
atmosphere models and on the new generation of three-dimensional (3D)
hydrodynamical models. We find that the non-LTE effects on the iron abundance
derived from Fe {\sc i} lines are very important, amounting to ${\sim}0.9$ dex
and to ${\sim}0.6$ dex in the 3D and 1D cases, respectively. On the other hand,
we find that non-LTE and 3D effects have to be taken into account for a
reliable determination of the iron abundance from weak Fe {\sc ii} lines,
because the significant overexcitation of their upper levels in the granular
regions tend to produce emission features. As a result such Fe {\sc ii} lines
are weaker than in LTE and the abundance correction amounts to ${\sim}0.4$ dex
for the 3D case. We derive also the oxygen-to-iron abundance ratio in the
metal-poor star HD140283 by using the O {\sc i} triplet at 7772--5 \AA and the
forbidden [O {\sc i}] line at 6300 \AA. Interestingly, when both non-LTE and 3D
effects are taken into account there still remains significant discrepancies in
the iron abundances derived from Fe {\sc i} and Fe {\sc ii} lines, as well as
in the oxygen abundances inferred from the O {\sc i} and [O {\sc i}] lines. We
conclude that the metalicity of this type of metal-poor stars could be
significantly larger than previously thought, which may have far-reaching
implications in stellar astrophysics.
