  We use high quality VLT/UVES published data of the permitted OI triplet and
FeII lines to determine oxygen and iron abundances in unevolved (dwarfs,
turn-off, subgiants) metal-poor halo stars. The calculations have been
performed both in LTE and NLTE, employing effective temperatures obtained with
the new infrared flux method (IRFM) temperature scale by Ramirez & Melendez,
and surface gravities from Hipparcos parallaxes and theoretical isochrones. A
new list of accurate transition probabilities for FeII lines, tied to the
absolute scale defined by laboratory measurements, has been used. We find a
plateau in the oxygen-to-iron ratio over more than two orders of magnitude in
iron abundance (-3.2 < [Fe/H] < -0.7), with a mean [O/Fe] = 0.5 dex (sigma =
0.1 dex), independent of metallicity, temperature and surface gravity.
According to the new IRFM Teff scale, the temperatures of turn-off halo stars
strongly depend on metallicity, a result that is in excellent qualitative and
quantitative agreement with stellar evolution calculations, which predict that
the Teff of the turn-off at [Fe/H] = -3 is about 600-700 K higher than that at
[Fe/H] = -1.
