Oceanus is the god of… wait for it… the Ocean.
Super creative, right? Anyway, Ocean is the river that the Ancient Greeks believed encircled the earth. (It was really just the Atlantic—maybe.) Because Scythia, the land where Prometheus is chained up, was believed to lie at the edge of the world, it makes sense that Oceanus (and his Chorus of daughters) would be hanging out there.
When we first meet Oceanus, he seems a little… bossy. He starts out by saying that he's "finally" reached Prometheus (285), as though the first thing he did when he heard about the punishment was hop in his car and speed over. And then he says he "share[s] the pain of your misfortune" (288), but he speaks in such formal, stilted words—"I assure you," kinship "compels me to do so," and "it is not/ in my nature to speak pleasant but empty words" (188-293)—that honestly? We're not convinced.
And then he tops it all off by saying "tell me what should be done to help you" (295): not, "how are you feeling, dude?," or "gee, you must have really believed in the cause," or even "is there anything I can do to help." It's like he doesn't stop to assess the situation before barging in with his big Titan-powers, ready to make a bad situation worse.
And he doesn't stop. He keeps needling Prometheus to let him, Oceanus, be Prometheus's "adviser" and tells him "keep quiet and don't speak too impetuously" (317-329), as though speaking impetuously isn't Prometheus's business.
Like Hephaestus and Hermes, Oceanus is just another of Zeus's tools—except he might even be worse. At least Hephaestus and Hermes are Olympians, so it makes sense that they'd be working for Zeus. But Oceanus is a Titan. He's betrayed his family and his cause (in Prometheus's mind) by sucking up to Zeus.
These two guys clearly don't get each other. Prometheus might understand Oceanus, but, in Oceanus's mind, Prometheus's words clearly do not compute. When Oceanus finally decides to quit the scene, he does so by making a half-hearted excuse about how his horse needs some rest. And that's Oceanus for you: terrified of offending anybody, he'll make up any excuse just to avoid "incurring enmity" (390-96).
We bet Prometheus is glad to see him go.