Colonization is one the major long term social movements on Oceana. It has been used to relieve social pressure in all Oceana's human and non-human societies. The prologue of colonization, adventuring, has also been used by all societies, human and non-human, on Oceana to reduce unrest. These are not the only reasons for colonization or adventuring, but are sufficient to justify the practice for now.
Colonization reduces social pressure in a number of ways. The biggest is by providing new lands for later born children that can not inheirit the family farm. Colonies are also a place for dissident groups to set up society the way They think it should be done. It does not matter that most such utopian colonies fail within a generation, usually with the death of most of the inhabitants. The third advantage of colonization is to provide a place to ship criminials to. It is dangerious, but less dangerous in the long run than execution or prison. The fourth source of colonists are retiring military volunteers. The military retirees get a big piece of farm land upon completion of service. The state gets a well organized group of tough people to expand the states influence.
Colonization is a fairly expensive and thus planned process if it has backing from a specific city-state. The typical method for the past 300 years or so has been that a company is formed, with appropiate bribes to government officials if needed. The company is granted one or more islands that are in that countries sphere of influence. The Company accepts colonists, charging for transportation and land so as to turn a profit. Colonists that can not afford to pay passage and buy land end up as the indentured servants of the land owners. Military retireess and proven adventurers are granted land or passage so as to provide protection for the fledgling colony. This has more in common with the early colonies in North America than with the spread west in the US or the creation of colonies in Africa during the 1800s. Oceana is an island world with lots of fast breeding monsters. These monsters, such as kobolds, orcs, goblins and such can be killed or enslaved, but can not be conquered & ruled like Africa or the Far East. As such, frontier military officials are unable and unwilling to spread themselves thin by expanding their prestigue, glory & wealth through conquest or colonization. Expansion to create more colonies occurs very slowly, and matches population pressures and need for resources instead. In general, expansion creates less security rather than more security for an empire.
Colonies are also a relatively easy way to create wealth for the already politically powerful back in the mother country. A colony would be created using the military, military retirees and captured monsters. The soldiers would get some land, but the big estates would go to the politically powerful in the mother country. The small farms would produce food for the colony, but the big estates would tend towards export crops such as olives, apples and longer term projects. This is similar to what happened in the Roman Empire on Earth.
In more ancient times colonization was handled by nobles of various city-states. Nobles would be granted lands at the edge of known map and be able to keep what they could hold. Many minor wars were fought over such divisions of land, usually with the older city-state winning by being able to hire better fighters and mages. This history of minor wars over recently developed islands lead to Oceana's version of feudalism. Instead of fighting the wars, the new colony leader would swear fealty to the nearby, powerful city-state or mother country of the colony. It helped that most of the nobles were related in some degree to each other, even in different city-states. Nobles from various city-states would gang together to grab a bunch of islands. The senior nobles would hand out land to both the lower classes, but also to the nobles. The nobles would also be in charge of the military, at least at first.
Adventuring provides an even more vital means of relieving social pressure. It provides and opportunity for the really aggressive to survive in a rough enviroment. Adventuring, unlike colonization, does provide more security usually. Adventuerers act to kill or enslave the monsters at little to no cost to the various governments. Governments don't have a stake in what the adventuers do unless the monster tribes manage to get organized.
In Oceana, many colonies of the older city-states have broken away from the originial founding government. Generally the originial government does not contest this, and has not contested this for the past hundred years. They just pull out their troops peacefully. It has usually not been profitable to stay without local support and taxes. People in many of the richer and older city-states are not interested in military service away from home. This is similar to how Great Britian pulled out of it's colonies after World War II. Exceptions do occur, especially if the taxes and profits from an island are still large. In those cases some areas have seen intense fighting between locals and their central government.
In the Border Lands, colonies and vassel islands are more often fought over, even to this day.
New colonies have to buy rights to islands; with the purchase comes some military protection. Failure to pay taxes for a number of years, and failure to support local forces will normally result in the withdrawal of troops. The effect is very similar to the British Commonwealth on Earth. Other causes for colonies to break away from their home countries is a form of civil war. Nobles or merchants that live in the colony do not want to pay taxes, or obey the central government's leadership. This has been the case for the past 100 years. Prior to the Monster Plague, uprisings were generally put down.
Most areas of Oceana have an intermix of cultures. Individual islands tend to be a single dominent culture, wether Greek, Norse or occasionally Egyptian, despite having settlers from many different nations. The drawing of colonists from a variety of island nations creates a more cosmopolitan atmosphere in a new colony. The colonists also have connections to a variety of originial governments. The lack of single loyalty makes it harder for any given country to claim and maintain control of the new colony. Groups of islands can often settled by groups from different cultures or countries. A result of this intermixing is to reduce the likelihood of major wars. There are too many small settlements that are near the settlements of other countries to make protection cheap. Trying to put a garrison on each island is to expensive, but would be needed in a wide scale war. Instead naval patrols keep raiders and monsters out cheaply.
Another group that created lots of colonies were many of the various sentient "monster" clans. Mostly they ran away from the humans and human allies or rival clans. The Kobolds, Goblins & Hobgoblins, and Orcs were all pushed into the western islands.
Colonization is a explosive movement. One or more groups will settle on a new island, and stay there for many generations. The next island will not be considered for colonization until the first island is between two-thirds and three-quarters full. Since the growth rate on different islands varies, there is normally a few islands about to be colonized in any given year. Also, islands will often draw colonists from numerous islands. Unlike colonizing of continental landmasses, the movement to another island is a very deliberate act.
See also the introduction to the Hammer's Slammers Handbook from http://www.miniwargames.com/ . It has an excellenet article on colonization that applies some what to Oceana.