The abundance of oxygen in 9 metal-poor stars is derived from high res
olution observations of OH lines in the spectral region 3138-3155 angs
trom. The same spectra were previously used to determine the beryllium
abundance (Gilmore et al. 1992). In addition, Mg, Ca, Ti, Cr, and Fe
abundances are derived from accurate equivalent widths of weak atomic
absorption lines in the 5100-6100 angstrom spectral region. Atmospheri
c parameters, T(eff) and g, are determined from the Stromgren photomet
ric indices b - y and c1. New accurate oscillator strengths for the Fe
i and Fe II lines allow an important check of these parameters as wel
l as the assumption of LTE; the Fe i excitation balance is consistent
with the photometric temperatures, whereas the Fe II/Fe I ionization b
alance suggests an over-ionization of iron amounting to about 0.15 dex
with respect to LTE. The derived abundance ratios between the alpha-e
lements and Fe are nearly constant in the metallicity range -3.2 less
than or similar [Fe/H] less than or similar -1.8. Any possible scatter
of Mg/Fe, Ca/Fe and Ti/Fe in the galactic halo is less than 0.06 dex
and an upper limit for the scatter of O/Fe is 0. 15 dex. This suggests
that the IMF was similar in different regions of the inner halo and t
hat the mixing of nucleosynthesis products was very efficient in galac
tic regions of typically globular cluster masses.