- "The Circle must be broken."
- ―An Ood
The Ood were a humanoid species with coleoid tentacles on the lower portions of their faces. They had no vocal chords and instead communicated by telepathy. This telepathy was perceived as song. They were originally a hive mind led by the Ood Brain. This kept control of the two brains in each Ood (a fore-brain embedded in their body, and a secondary hind-brain connected by an umbilical cord-like connection to their faces, usually held in their hands). The hind-brain enabled them to connect to a single large brain, which coordinated individual Ood. The Ood Elder had a larger, external fore-brain.
When in the servitude of Humans, their hind-brain was removed and a translation sphere fitted in place, not only to enable speech, but also to limit their telepathic communications field at "Basic 5" which cut them off from the Ood Brain, making them willing to serve and incapable of surviving on their own. The colour of an Ood's eyes changed with the level of telepathic activity, normally glowing bright-red but turning to white when not connected to a hive mind. Humans, who only knew the Ood in their telepathy-disabled form, misinterpreted the presence of red eyes as a disease, known as red-eye.
The Ood had long lifespan, with Ood Sigma living 100 years after the Human enslavement ended and the Doctor being surprised at this, thinking he should have been much older.
There seem to be no visual distinction between male or female Ood, which would suggest that they are hermaphrodites. On the other hand, when Donna Noble called an Ood "it", Tenth Doctor corrected her, saying she should call him a "he".
The Ood were a peaceful race originally. In the 3800's, Humanity discovered the Ood, enslaved them, and used them to perform menial tasks throughout the three galaxies of the Second Great and Bountiful Human Empire. The external hind-brain of the Ood was extracted by Ood Operations and replaced with their translator globes. With their connection of the Ood Brain severed, they followed the orders of Humans. Ood Operations kept this procedure a secret from the rest of humanity, spreading the belief that the Ood were naturally servile and offered themselves.
By the 42nd century, it was claimed that every Human had an Ood servant. They were generally barely even regarded as being alive, and were considered to be expendable. Despite this, an activist group, the Friends of the Ood lobbied for the cause of their freedom.
In 43K2, the Tenth Doctor and Rose Tyler came to Sanctuary Base 6 on Krop Tor, where the Ood were used for menial labor. Unlike the Humans or the Doctor, the Ood knew of prophecies regarding the Beast which the expedition would later discover. The empathic nature of the Ood seemed to make them susceptible to psychic possession by the Beast, who made the Ood on Sanctuary Base 6 into his "Legion" in order to besiege the Human party of the Walker Expedition. They were defeated when Danny Bartock, the expedition member in charge of them, broadcast a telepathic flare which reduced their field to Basic Zero. This created a "brainstorm" which caused them to collapse. However, their telepathic field began to reassert itself after a time. When Krop Tor was sucked into the black hole, the Doctor was unable to save any of the Ood on the base, who had just been freed of the Beast's control. All perished but were given posthumous honors.
In 4126, the Doctor and Donna Noble visited the Ood Sphere by happenstance. They found Ood Operations, and decided to learn the origins of the Ood and what or who had turned them into a race of slaves. At the same time, the "red-eye" disease was becoming more and more common. They discovered that the disease was caused by the Ood Brain's subconscious reasserting itself over the Ood thanks to the efforts of a member of "Friends of the Ood" who had infiltrated the company secretly. Ood Operations' head, Klineman Halpen was unwillingly transformed into an Ood as a culmination of him taking hair tonic deliberately contaminated by his own Ood servant. The Doctor deactivated the field that was suppressing the Ood Brain's telepathy and it sent out a song that was heard by everyone across the three galaxies. Humanity, now knowing the truth about their Ood slaves, returned them to the Ood Sphere, where they were set free and allowed to live in peace.
Circa 4226, the Ood's development had been unnaturally enhanced, allowing them to build an entire civilization and extended their mental powers. At the same time, the Ood had nightmares of The Master (Harold Saxon) a mysterious face. With this, they sent Ood Sigma to 2059 to contact the Tenth Doctor. Doctor Who The Waters of Mars He eventually arrived on the Ood Sphere and telepathically connected to the Ood Council. When he saw the Master, he immediately returned to the 21st century to investigated. The Ood watched him and when he finally began to die, they telepathically sang to him Vale Decem, which translates to "Farewell Ten".
- Though not stated, the unnatural developments may have been from changes in time from the Master Race or the actions of the Time Lords.