Oceania was a theme park created by Grant Walker.
Grant Walker, creator of several theme parks, came up with the idea of a theme park partially underwater in the middle of the ocean. He called it Oceania and soon, the theme park expanded enough to be a city by itself. Walker decided to use Oceania as a base of operation in his evil scheme of turning the earth into a vast frozen wasteland and he would be safe among people chosen by him to stay in Oceania.
However, he needed Mister Freeze's help to achieve his mission and he kidnapped the scientist from Arkham Asylum, which caught Batman's attention. Batman and Robin learned from Karl Rossum about Walker and Oceania and soon, they learned about Walker's plans. Walker blackmailed Mister Freeze and forced the man to work for him under the claim that he could save Freeze's wife, Nora.
Walker was ready to start the planet's transformation, and Batman made Mister Freeze realize the mistake he has done, and together, Batman, Robin and Freeze destroy the main control board of Oceania. Freeze overloads the main power core and the whole city of Oceania starts turning into a giant block of ice. The generators explode creating fire all over the city. Walker tries to stop Freeze, but using his cold gun, Mr. Freeze traps him in a block of ice from head to toes.
In the end, Oceania was utterly destroyed, leaving nothing but floating icebergs in the sea.