Oceanus was a Titan god of enormous river called Okeanos which was believed to be encircling the world and was connecting this world to other realms, such as heavenly realms from which the gods came and underworld where the souls of the dead lived.
He was a personification of all the salt water of the world, more precisely Mediterranean Sea, Atlantic Ocean and Indian Ocean which were the most familiar to ancient Greeks at the time, and all the fresh water that includes rivers, lakes, streams and rain.
Oceanus was originally the source of all the water. However, when geography became more known and accurate to the Greeks, he was placed to represent more stranger and unknown waters of Atlantic and Indian Ocean while Mediterranean Sea was ruled by a younger god Poseidon and rivers, fountains, streams and rain waters were controlled by the titan's sons and daughters.
Oceanus was the first descendant of Uranus and Gaea and was married to Tethys.
Okeanos' wife Tethys was envisioned distributing his nourishing waters across the earth through subterranean aquifers. Together they gave birth to three thousand river gods and Oceanids, known as nymphs of rivers, lakes, streams, fountains and wells which never ran dry.