An Over Soul is basically making the shaman and spirit ally's form an extension, a body, a weapon, and more out of furyoko (mana).
Oversoul is the ability to integrate the spirit into a medium such as a sword, skeleton, or any objects a shaman desires (the object that the spirit integrates is called the spirit medium). This is the opposite of integration (spirit unity) which is the shaman integrating the spirit with their body. However, to achieve the best abilities of the spirit, a shaman must integrate the spirit into an object the spirit is familiar with. For example, Yoh integrates Amidamaru into a katana and the katana is the most appropriate weapon for a samurai.
Flaws of the oversoul ability is that it consumes a lot of energy, furyoku/ mana. When a shaman's will falters, the over soul becomes weaker. Also, over souls cast no shadow. This is useful in battle to determine a doppelganger over soul.
*Note: This is from the Shaman's concept page. Mana and Furyoku are used interchangeably.
Characters who don't use over souls: Jun Tao, Keiko Asakura, Kino Asakura, Tao Ran, and Tao Ching. Redseb and Salerm are excluded because their golem uses an internal mana drive. Exceptions: Manta has an over soul in the anime version only.
A shaman integrates the spirit into one spirit medium for various purposes: transportation, forming weapons, and more. A shaman uses their mana/ furyoko to power it.
Every shaman who integrates a spirit into a spirit medium. This does not include Redseb and Salerm's golem since it uses an internal mana drive.
Chocolove integrates Mic into his hands and legs while still integrating Mic into the claws (spirit medium).
This occurs when integrating a spirit into the spirit medium and then integrating that oversoul into another spirit medium.