After sinking Atlantis to save it in the Obsidian Age, Orin was deposed and banished from Atlantis. The sorcerer caste became the rulers of Atlantis under Haken. Orin's telepathy with marine life was blocked and he was run out of Atlantis and, after that, the whole ocean. He was rescued by the Lady of the Lake, who gave him his magic hand. After that, he wound up at a lighthouse in Ireland with an old lighthouse keeper named McCaffrey and a young cop lady whose name I can't recall because she was unimportant after the first arc and totally forgotten after Pfeiffer came along.
Orin was content to just deal with his situation, but Tempest came calling to show him the problems the sorcerers were causing. Dolphin and Cerdian were basically under house arrest and Mera was kept magically sedated, so even though she was the true ruler in Orin's absence, power fell to the sorcerers. Aquaman still turned a blind eye, claiming he had problems of his own.
Eventually, Aquaman learned that his water hand was never supposed to be used in anger or there would be consequences. Unfortunately, he'd already turned it into a big spiked ball and punched someone in the face. The consequences turned out to be the Thirst, an ancient dude who wanted to devour all water, including the Secret Sea. Aquaman teamed up with Black Manta and defeated the Thirst (along with Black Manta, after he inevitably betrayed Orin), and in doing so liberated Atlantis somehow (it's been a while since I read it). He left Atlantis in Vulko and Garth's care and headed off to Sub Diego with basically nothing in the way of transition, as has become the norm for Aquaman's current series.
Aquaman heard that a piece of San Diego had sunk into the ocean and went to help, but when he tried saving citizens by bringing them to land, they started suffocating. Everyone except Lorena could breathe only water somehow. Orin returned the people to the underwater portion of San Diego and then he and Lorena set about trying to restore some order while also trying to figure out how the whole thing happened. Orin restores order with the help of a cop who was mutated into a manta ray-man, Lorena, and Koryak. He figures out that it was a scientist named Geist who turned Sub Diego's people into water-breathers by introducing mutagens into the water supply for months before the sinking. Geist turns out to be a small fry, though, and Orin keeps him locked up and working on a cure or whatever other scientific assistance he needs while he goes after the bigger fish. The mutagen Geist put in the water supply turns out to have come from Orin's own DNA, but before Orin can look into the company that holds the patent on his genetic code, Vulko comes calling as an emissary from Atlantis, intent on inviting the Sub Diegans to join the Atlantean people in Poseidonis.
Lorena goes with him, and Aquaman follows shortly after when he learns that Mera is still under the sorcerers' spell. Aquaman and Tempest fight for some reason that still makes no sense to me today, and then Tempest harnesses the magical powers of Haken and the other sorcerers to try and free Mera. Unfortunately, Infinite Crisis rears its ugly head and the big magical fireworks show draws the Spectre's attention. He smushes Atlantis, Koryak and Vulko die, Tempest and Lorena go MIA, Mera ends up leading a tiny band of Atlanteans and Sub Diegans, and Orin ends up turning into a squid man by some unknown means and getting replaced in his own book by an angstier carbon copy of his Golden Age self. That's pretty much where we are today.
