Oceanus is the Titan personification of the ocean, which the ancient Greeks believed to surround the world in its entirety. He is a son of Gaia and Uranus and resembles the ocean that is far away (so not the Mediterranean sea that is central to Greek culture).
He married his sister, Tethys, and together they would raise the Oceanids: water nymphs that protect the oceans. Through their union, they also gave birth to Potamids and Nyads. These were river nymphs and gods that protect the rivers and springs. Finally, Oceanus is also the father of Triptolemos. This demi-god would later have an interesting run-in with Demeter.
Unlike his siblings, Oceanus did not participate in any of the battles between Titans and Olympians. Instead, he secluded himself to his own private realm. His offspring (and their offspring) did participate in the war, but they fought on the side of the Olympians.