The hypothesis of an oxygen limited thermal tolerance was tested in the Antarctic teleost Pachycara brachycephalum. Using flow-through respirometry, in vivo 31P-NMR spectroscopy and MRI, we studied energy metabolism, intracellular pH (pHi), blood-flow and oxygenation between 0 and 13°C under normoxia (PO2: 20,3 to 21,3kPa) and hyperoxia (PO2: 45kPa). Hyperoxia reduced the metabolic increment and the rise in arterial blood-flow observed under normoxia. The normoxic increase of blood-flow levelled off beyond 7°C, indicating a cardiovascular capacity limitation. Ventilatory effort displayed an exponential rise in both groups. In the liver, blood oxygenation increased, whereas in white muscle it remained unaltered (normoxia) or declined (hyperoxia). In both groups, the slope of pHi changes followed the alpha-stat pattern below 6°C, whereas it decreased above. In conclusion, aerobic scope declines around 6°C under normoxia, marking the pejus temperature Tp. By reducing circulatory costs, hyperoxia improves aerobic scope but is unable to shift the breakpoint in pH regulation or lethal limits. Hyperoxia appears beneficial at sublethal temperatures, but no longer beyond when cellular or molecular functions become disturbed.