The angst of Oceanus joining the Titans in PJO is so unexplored.
While he stayed out of the first titan war for the most part, his daughters Metis & Styx were definitely Olympian loyalists, with Metis even being the one who made the potion Zeus served to Kronos (making him throw up his kids) but his daughter Clymene's sons with Iapetus were divided.
After the Titanomachy, he & Tethys raised Hera, who loves them dearly.
His daughter Metis is married to the new king Zeus, so everything should be going great for their family right? Except no, his son in law does the same thing to his pregnant daughter that his cannibal brother did to Zeus's siblings, and unlike the children of Kronos, his daughter wasn't lucky enough to escape her cannibalizer.
His granddaughter is born eventually, but she sticks by Zeus. She's even his favorite with Zeus getting credit for creating her alone. She never mentions her mother, Oceanus's daughter, it doesn't seem like she even cares.
And Hera is now married to Zeus, she loves him, so he tries to support her, only to see her heart get broken over and over again. Hera loses her patience when it comes to Leto, every god including her own mother betrays her, but Oceanus & Tethys still stick by her.
Maybe Oceanus is grateful when Hera spares his granddaughter Maia from her wrath, despite Maia being just as guilty of having an affair as Leto? But when it comes to another one of his granddaughters, Io, she's not so gracious, and unlike Leto & Maia, Io never even accepted Zeus's advances. She was assaulted, by the same cannibal who ate his daughter. Fortunately Io survives, but Hera made it clear she's not making exceptions for his family anymore.
Yet Oceanus still loves her. She's just as much a daughter to him as the others, and she loves him & Tethys, she comes to them with tears in her eyes constantly. Zeus's actions sucking out her joy and warmth more and more each time they see her. Hera has abandoned her kindness, only hurt is left in his once affectionate foster daughter.
She makes him & Tethys swear to not help Callisto, he loathes doing it, but if he betrays Hera the same way Rhea did, it would destroy her, she would never trust him again. Everyone else has always chosen Zeus over her, even though it was him who was the source of Hera's misery, and as he only ever admitted to himself, the source of his own misery as well.
Kronos comes back, & the rest of his brothers.
Hecate, once an ally of Zeus, joins the Titans. While beloved by Demeter, Hecate is just as loving of a mother as the goddess she served, and there is no place for her children. She brings with her some allies from the Underworld in Nemesis, Morpheus, & Melinoe.
His grandson & nephew Prometheus expects loathing from his father as he sided with the Olympians, but Iapetus only embraces him upon reunion. Oceanus sees his brother's eyes burn with rage once he learns Zeus's brutal punishment & torture of his son.
The desire for revenge by those locked in Tartarus are clear, but Zeus has hurt them in more ways than one. Hyperion has lost Helios & Selene and Koios has lost Asteria just as permanently as Oceanus has lost Metis, he grieves with them. Their children fought for Zeus and the Olympians over their own kind, the Titans, just to be brutalized, abandoned, & tortured when they were no longer wanted.
People sing praises to the Olympians, but all Oceanus can see is the pain they caused his family, and all this time, he did nothing.
He didn't help dispose of his abusive father, he didn't stop Kronos from eating children, he stood by as his own children & grandchildren fought each other in the first Titan war, he didn't retaliate for the murder of Metis, nor for the injustice faced by Prometheus, not even when his own daughters & granddaughters were getting assaulted by Zeus, Poseidon, & their spawn, and he's tired of doing nothing, so he joins.
All he makes Kronos promise him in turn is that Hera will remain unharmed under Kronos's new reign.
She's hurt his family, and that hurts him, but if Zeus was gone, there would be no women she would have to resent.
