Oceana was settled by groups from other worlds that arrived by unknown means. Most of the stories involve some sort of bright light and a sense of motion. Groups have arrived with everything from just their clothes to entire villages including buildings, animals and crops.
There have been 6 major groups of transdimensional travelers. These are the Egyptians, Elves, Dwarves, Greeks, Jews and Neko-jin. All of these groups have at least some written history that explains how their race came to be on Oceana. These six groups have formed the backbone of Oceana's civilized and settled world.
The monsters and some other groups do not have a written history. These include the Orcs, Kobolds, Goblins & Hobgoblins, Bugbears, Gnolls, Ogres and Giants. It is fairly obvious that these tribes didn't come into being on Oceana since they don't have any real old ruins, nor did they have sea travel until the Greeks found them. In addition to the land based monster tribes, there are also ocean dwellers that don't have a written history. These include the merfolk, lizardmen and such.
Besides the 6 major groups, there have been much smaller groups and individuals that have ended up on Oceana. Most of the time individuals are just absorbed into Oceana society, often as slaves. Small groups don't have a major social impact directly, but they have been know to introduce new technology and concepts to Oceana.
Around 1200 to 1400 CE a herald from Europe on Earth.introduces the concepts of noble ranks that is associated with feudalism. He can't introduce actual feudalism, which sort of already existed. What he was able to introduce was noble titles and gradations as a form of social one-upmanship.