Feudalism on Oceana is very different than feudalsim and associated manoralism on Earth. The major difference is that on Oceana, feudalism is based around the polis, or city, rather than the isolated manor of Europe. In addition, feudalism on Oceana wasn't the result of the collapse of civilization, it was a by product of the growth of human civilization. Unlike Earth, Oceana's version of feudalsim worked with a market economy. The nobles were the tax recipients along with the military and legal power. While there is some manoralism, it isn't the basis for Oceana's feudal society.
Oceana's version of feudalism arose as a means of limiting warfare among humans and as a response to the monsters who raided human settlements. The basic process was tied to adventurers and colonization. The details also depended upon if the island being colonized was close to an existing city-state or far away from all of them.
If the island selected for colonization was far from existing city-states, the adventurers would just grab the island and make preparations for starting a new colony, such as clearing it of monsters. Adventurers that choose distant islands usually refer to each full member of the party or their descendents as king(s) or princes of that island by virtue of having lead the colonization effort. Their followers and henchmen would be lessor nobles of that island. This is one reason a large number of islands have more than one king, or princes. The party would then recruit settlers to inhabit the island. Alternatively, the adventure party might have been hired by a group of colonists to prepare an island for them. The party, or individual members and their descendents would have an oath of fealty with new city to provide protection and governance in exchange for being the rulers.
The colonists would found a city, albeit a small one, surrounded by farms and procede to settle in. The colonists would insists on political rights from the adventure party as a condition of settling on the new island. The amount of rights depended upon how rich the settlers were and how powerful the adventurers were. In general the party & descendents would own the island and rule the overall island. The settlers though would have the city, and run the city by a variety of means. The king of the island would have final jurisdiction over the city, and live in the city, but not be involved in day to day affairs.
In cases where the adventurers were hired to prepare the island for colonization, it is more common for them not to become nobles. Instead they do the job, get their money and leave the island alone.
If the island was closer to an existing city-state, the use of a form of feudalism was additional a means of keeping the peace. The adventurers would be granted a piece of land on a nearby island, or the same island, in exchange for swearing fealty to the noble, or city-state govenment that had laid claim to the island. Adventurers would take this option to avoid having to fight the other local islands for jurisdiction and to have access to local ports. A new colony was often very vulnerable at first. This was not just to attacks on the actual colony, but to being denied access to local resources or piracy by local lords.
Feudalism also worked, sort of, with the various monster tribes. The leader of the tribe would be willing to swear featly to the conquering adventurers to save their tribe. This fealty though would have to be to a specific person, not to say a council that changeed every few years.
Magic also made feudalism more acceptable. This would be true with both Oceana's version and Earth's European version. Powerful mages, clerics and magical items for them and fighters puts them a huge step above the common soldier. Such powerful classes are easier to control with a feudal bond, and would be very hard to cow or control by brute force. Nobles recruit such heros and grant them fiefs as a means of control over both the adventuers and to off-set other noble's powerful mages, etc. Part of the payment is land and peasants to work the land.