Oceanus, like every Eternal, was created by the Celestials one million years ago. 400,000 years later, a civil war erupted between two factions of the Eternals: the side of Kronos, who cherished piece and non-interference, and the side of Uranos, wanting to use their powers to dominate the planet. Oceanus joined Kronos' side and together they defeated Uranos and his followers, who were banished from the Earth.
Still, some of the exiled Eternals met their doom in the colony of Titan and were resurrected on Earth, in the Exclusion, where they were mind-wiped. After that, they joined Oceanus, who had retired to a vigil of Deviants. Together, they became known as the Oceanic Watch.