[Alneyan]

You will have to download the Oceania mod from Shrapnel download page, extract its contents to your mods directory, and select the Oceania mod before creating a Pretender. Once you have started a game with this mod, it will be automatically switched on when you play this particular game.

Oceania does not seem to be available under the special themes of Atlantis (unless I badly need to have my eyes checked), and instead replaces the whole nation.