We use high quality VLT/UVES published data of the permitted O i triplet and Fe ii 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 Ramírez & Meléndez , and surface gravities from Hipparcos parallaxes and theoretical isochrones .
A new list of accurate transition probabilities for Fe ii lines , tied to the absolute scale defined by laboratory measurements , has been used .
Interstellar absorption has been carefully taken into account by employing reddening maps , stellar energy distributions and Strömgren photometry .

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 .
The flat [ O/Fe ] ratio is mainly due to the use of adequate NLTE corrections and the new IRFM temperature scale , which , for metal-poor F/early G dwarfs is hotter than most T _ { eff } scales used in previous studies of the O i triplet .

According to the new IRFM T _ { eff } 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 T _ { eff } of the turn-off at [ Fe/H ] = -3 is about 600-700 K higher than that at [ Fe/H ] = -1 .
Recent determinations of H \alpha temperatures in turn-off stars are in excellent relative agreeement with the new IRFM T _ { eff } scale in the metallicity range -2.7 < [ Fe/H ] < -1 , with a zero point difference of only 61 K .