So days NASA, after measuring it outgassing:
"We were able to measure the shape of the cloud, estimate the amount of water it contained and the rate it would be destroyed and produce oxygen," says Esposito. The amount of water they saw, about a million tons, was exactly that needed to provide a cloud of oxygen like the one they had first observed near the E ring more than a year earlier. "This was a most pleasing result," says Esposito. "We measured two new distinct phenomena and found that they fit together."
The mystery of the atomic oxygen was solved. At the same time, its source, the diminutive Enceladus revealed itself to be completely different than the cold, dead icy moon it should have been. Small as it is, it has an internal heat source and is geologically active. Its geysers throw out enough water vapor and ice to maintain the moon's atmosphere, feed the vast E ring, and decompose into clouds of oxygen like the one first spotted by Cassini on its way to Saturn.
So there you have it: small moon, big story.
I wonder if Earth has ever been fed oxygen by another solar body?