The oxygen abundance of HE 1327-2326

From a newly obtained VLT/UVES spectrum, we have determined the oxygen abundance of HE 1327-2326, the most iron-poor star known to date. Ultraviolet OH lines yield one-dimensional LTE abundances of [O/Fe]_OH = 3.7 (subgiant case) and [O/Fe]_OH = 3.4 (dwarf case). Using a correction of -1.0 dex to account for three-dimensional (3D) effects on OH line formation, the abundances are lowered to [O/Fe] -2.8 and [O/Fe] = 2.5, respectively, which we adopt. Without 3D corrections, the UV OH-based abundance would be in disagreement with the upper limits derived from the O I triplet lines: [O/Fe]_trip < 2.8 (subgiant) and [O/Fe]_trip < 3.0 (dwarf). We also correct the previously determined carbon and nitrogen abundances for 3D effects. Knowledge of the O abundance of HE 1327-2326 has implications for the interpretation of its abundance pattern. A large O abundance is in accordance with HE 1327-2326 being an early Population II star that formed from material chemically enriched by a first-generation supernova. Our derived abundances, however, do not exclude other possibilities such as a Population III scenario.