Oceania, as seen in Extreme-G 3, is Paradise City from Burnout Paradise and Pacific City from Crackdown in the far future, as well as Big Blue from F-Zero GX before it became an industrial complex
This is a little whack, but hear me out: Paradise City's three urban districts got carried out to sea, where they became Pacific City. Then it became the city in the sea known as Oceania. Most of the living structures are gone, or deep underwater. The place is more of a tourist destination than anything else. They decided to build a race course to draw in more visitors, just because we're greedy like that. It later gets converted to an industrial complex in a couple hundred years, becoming Big Blue, as seen in F-Zero GX. The first game's backstory
During the Earth's scramble for space, the industrial strain required for an interplanetary society overpolluted and overpopulated the world, turning whole cities into polluted slums (the City tracks) and forcing corporations to strip the planet of its minerals at a never-before-seen rate (the Mines tracks). The discovery of an ancient alien civilization on a backwater world (the Desert tracks) was such a massive shock to humanity that the only way to distract the masses from the prospect of an imminent "great filter" was a newer, faster, ultra-violent sport: Extreme-G.