The Oomori clan is a group of shapeshifters, to put it simply. It is said that its founder learned how to utilize chakra to shift forms into that of many different animals, and he taught the skill to those willing to learn. As the clan evolved over time, it slowly turned into a number of small individual groups, each aligning to a specific animal that they are initially drawn to as children. Because the ability has been so thoroughly cultivated within the clan, people born to it are able to shift their bodies into animals from a very young age without any training.
Younger Oomori have little control over their innate shifting powers, so many youth can be found in partial-forms with undisguised animal ears and tails. After learning initial control over the shifting as well as survival skills, they are sent to the appropriate group within the clan (or they choose to go off on their own). Older Oomori who choose to survive with their animal tribe will teach the young and care for their elderly. While it is not unheard of to see Oomori older than the age of 10 that do not share the same animalistic traits as their tribe, it tends to be rare.
The Oomori, starting near the age of 2 or 3, will be drawn to a specific animal and tend to show the traits of it through their unintended transformations. This may happen regardless of exposure to that animal (ie: a child who has never seen a bear may still be drawn to it). While this animal is their primary focus, some Oomori may choose to explore other animals. These secondary animals that are explored tend to be less complete transformations, especially if the Oomori chooses to transform into many different animals.
Transformations can somewhat be mixed, but not extensively. Human limits are somewhat respected, as an Oomori cannot shift into sizes larger than an elephant, nor can they change into a size smaller than a modern house cat. However, the size of the Oomori upon transformation can vary according to the user. It requires a burst of chakra to shapeshift, but very little to maintain it. Transformation is noted to be painful, but the Oomori are extremely used to it.
Each of the tribe heads will keep in regular contact with each other, and it seems that the Oomori have chosen to avoid inner conflict for the sake of avoiding extermination during the Clan Wars. This peace has extended into the beginning of the Era of the Hidden Villages for the 'Core Animal' tribes.