Oceanus was the divine personification of the world ocean, a magical ocean-stream that circled the earth at the equator. Sometimes he represented all bodies of salt water, sometimes just the Atlantic ocean. He was a huge, muscular snake man with a beard and horns. Instead of a humanoid lower body he had a snake bottom or in some depictions a fish body. He’s one of the few Primordial Titans (some of the gods kind of belong in both categories) that did not get involved with the Titanomachy.
Tethys was often listed as Oceanus’ consort. She was the daughter of Uranus and Gaia. She was mother to the Nile, the Alpheus, the Maeander, and about three thousand daughters called the Oceanids. Sometimes she and Thalassa or Thetis get swapped around in stories, but Tethys is definitely her own goddess. With the exception of raising Hera, she’s not depicted much in any of the known Greek myths or paintings, but when she is described it’s as an ancient woman. An extremely powerful ancient woman. Once, just to make Hera happy, she rearranged the constellations by sheer force of will so that Ursa Major and Minor never dipped below the horizon. One of the moons of Saturn is named for her, as is the preheistoric ocean.
Ophion and Eurynome don’t factor into many versions of the Greek myths, but in a few they ruled the universe before Cronus and Rhea. Eurynome was a daughter of Oceanus and Ophion was a giant. They fought against Cronus and Rhea and lost.