Oceanus

Oceanus is the eldest of the Titans and the father of the waters, the son of Ouranos and Gaea. The Romans called him Ocean. Oceanus is the husband of Tethys who bore him the Oceanids. He is the predecessor of Poseidon as the ruler of the seas and lord of water in all its forms, be it snow, rain or salty water.

History
Oceanus' consort is his wife Tethys, and from their union came the ocean nymphs, also known as the three-thousand Oceanids, and all the rivers of the world, fountains, and lakes. From Kronos, of the race of Titans, the Olympians gods have their birth, and hera mentions twice in Iliad book her intended journey "to the ends of the generous earth on a visit to Oceanus, whence the gods have risen, and Tethys our mother who brought me up kindly in their own house."

In most variations of the war between the Titans and the Olympians, or Titanomachy, Oceanus, along with Prometheus and Themis, did not take the side of his fellow Titans against the Olympians, but instead withdrew from the conflict. In most variations of this myth, Oceanus also refused to side with Kronos in the latter's revolt against their father, Uranus.

Eventually Oceanus retreated to the deep sea and was succeeded by Poseidon as ruler of the seas.

The Last Olympian
Oceanus was neutral in the first war, but Kronos convinces him to fight against the gods in The Last Olympian, more specifically Poseidon. He enlists Aigaios as well as many ancient monsters against Poseidon, and they battle constantly, leaving Poseidon weakened. It should be noted that Amphitrite is his daughter, and that she joins forces against her father along with her husband Poseidon. Oceanus and his forces attack Poseidon underwater, making his palace endangered and also making him unable to aid the Olympians fight the monster, Typhon. After Kronos was defeated, he retreated back into the depths.

Appearance
Oceanus is often depicted as having the upper body of a muscular man with a long beard and horns (the horns are often represented as the claws of a crab), and the lower torso of a serpent. In The Last Olympian, he was describe by Percy as a merman larger than the rest with the horns of a bull. He appeared to be riding a chariot pulled by crayfish with a serpent as a weapon.

Trivia

 * As Titan of the great earth-encircling river Oceanus, he is the source of all (fresh) water on Earth.
 * He was equated with Pontos, (protogenos of) the sea.