The hypolimnion of a quarry pond at Ottoville, Ohio, was oxygenated during the summers of 1973 and 1974. The highest hypolimnetic oxygen concentrations attained were 8.0 mg/1 in 1973 and 21.5 mg/1 in 1974; thermal stratification was maintained both years. The 21.5-mg/l oxygen concentration is the highest value yet reported for a hypolimnion and constitutes an upper limit controlled by pressure, temperature, and oxygen demand. Degassing was observed as the high value was reached. Hypolimnion concentrations (of N2) were calculated for different injected mixtures of O2 and N2 gases.