Io is similar to Prometheus because Zeus has horribly punished her, too. What makes Io different is that she has clearly done nothing wrong to the god. In fact, her only crime was that she obeyed an oracle saying that she had to do it with Zeus.
Thanks to Io, we can see how special it is that Prometheus actively challenges Zeus instead of running around complaining about being a cow. (Well, we can't really blame Io for that one.)
Oceanus thinks Prometheus should just apologize and recognize that things have changed. Prometheus, however, thinks that trying to make nice with Zeus is betraying the cause; he'd rather hang out in the bowels of the earth for centuries than reach a compromise.