Most, if not all of the transplanted groups brought their own religion with them. Over time though, the originial religions were replaced by an Oceana specific religion. Oceana, unlike Earth, has visiably active gods. When the followers of the various Earth gods (or other planets) arrived on Oceana they initially continued to worship the gods they brought with them. After a short time for most groups, one or more Oceana gods announced their presence to the new comers. Conversion usually was fairly quick since the Oceana gods filled similar roles, but were able to grant their new followers clearly visiable magic that non-Oceana deities could not.
Religion is universal in that all humanoid races believe in at least the existance of the same gods. Each god has particular followers based upon country, location, and occupation. People make offerings to many gods based upon their needs. If they are about to travel they will make offerings to Ogefea, god of travelers. This would be true even if their primary occupation was a metal worker who makes offerings to Melytec. This is also true even of clerics. While a cleric of Ogefea will offer to Ogefea every day, they will also offer to non-opposed gods as needed. Clerics will also attend as laity the temple services of other gods.
Almost all individuals will have a particular deity that they pray make offerings to. They will tend to follow the precepts and practices of that god. While a lot of this is similar to standard D&D alignment, each god will have slightly different standards, requirements and restrictions. In exchange for such offerings and following the precepts, individuals expect to have better luck doing things controlled by that deity. How well they follow their god's requirements has no effect on their afterlife normally.
The major exception to this is the Jews who are heno- / monotheisitic. They worship only one god and follow the precepts of that one god.
The Gods of Oceana are well known to the various inhabitants of Oceana. While most people, human or otherwise, do not worship more than a few gods, or more than one, they know that many gods exist. The various gods have proven themselves to exist on numerous occasions, including in recent times. As such, there are no atheists on Oceana. There are many who are only marginially religious, doing the bare minimum of required offerings and prayers.
The gods are "active" as is normal in a D&D campaign world. The gods perform miracles and show proof of their existance to large numbers of people on a sufficiently recent basis to prove their existance. The gods rarely if ever directly intervene directly in human affairs though. They do offer their followers spells and such on a daily basis. They will offer special powers to their most loyal and followers. The gods, especially new gods offer visions to various people, including non-clerics.
The true nature of the gods is unknown to the inhabitants of Oceana. When the gods have shown up, they have taken various forms including human, animal, or other such as fire. They are generally seen as super powerful human or human-animal creatures. The source and nature of divine and arcane magic is unknown. It is known there is a difference, but not what the reason(s) for the differences are.
The creation story of Oceana and it's gods goes as follows for non-Jews.
Out of the primordial Chaos of waters was formed Oceana and it's islands, and the various gods. Most of the gods are not related to each other unlike the Greeks, Japanesse, etc.
While the Jews have the story of Genisis, they know that the story took place on another earth. While Adoni / Elohim created the universe, other people can have other beliefs without disturbing their belief that Adoni created it.