The effect of partial oxygen pressure on growth of Neochloris oleoabundans was studied at sub-saturating light intensity in a fully-controlled stirred tank photobioreactor. At the three partial oxygen pressures tested (PO2=0.24; 0.63; 0.84 bar), the specific growth rate was 1.38; 1.36 and 1.06 day-1, respectively. An increase of the PCO2 from 0.007 to 0.02 bar at PO2 of 0.84 bar resulted in an increase in the growth rate from 1.06 to 1.36 day-1. These results confirm that the reduction of algal growth at high oxygen concentrations at sub-saturating light conditions is mainly caused by competitive inhibition of Rubisco. This negative effect on growth can be overcome by restoring the O2/CO2 ratio by an increase in the partial carbon dioxide pressure. In comparison to general practice , working at partial O2 pressure of 0.84 bar could reduce the energy requirement for degassing by a factor of 3–4.