Provided is an oxygen-generating composition capable of yielding a sufficient oxygen release rate and maintaining a stable water quality environment even at a low calcium peroxide content. Also provided is a method for manufacturing the oxygen-generating composition. An oxygen-generating composition comprising a molded article containing 2-20% calcium peroxide, 2-30% calcium carbonate, 2-30% calcium hydroxide, and 30-80% calcium dihydrogenphosphate, wherein the oxygen-generating composition is characterized in that the calcium dihydrogenphosphate contains from 10% to 40%, as calculated in terms of the weight of the calcium dihydrogenphosphate, of calcium dihydrogenphosphate particles having a mean particle diameter of 150×10‒6‒800×10‒6m and a maximum particle diameter of less than 106×10‒6m. Also, a method for manufacturing the oxygen-generating composition.
