On a water-world, with no easily accessible crust, the water itself could produce free oxygen by photolysis alone. On our world photolysis does not produce enough oxygen to be detectable, but on a waterworld with an ice mantle there would be no process to remove it except dissolving it in the water itself.
If we ever discover any oxygen-rich worlds we will have to discriminate between worlds with biotic oxygen and others with abiotic oxygen like waterworlds.