Poseidon and Oceanus were both powerful gods in Greek mythology associated with water. Oceanus ruled the river Oceanus while Poseidon ruled the seas and other bodies of water. They had different wives and children - Oceanus's wife was Tethys while Poseidon's included Demeter and Amphitrite. Though they both once ruled water sources, Poseidon gained dominion over more areas after the gods defeated the Titans in war. Oceanus remained calmer and avoided conflict while Poseidon was sometimes depicted as angry, reflecting the roughness of the seas.