The Ood, also known as Oodkind, were a gestalt race of telepathic humanoids who originated from the Ood Sphere.
Biology
The Ood were a humanoid species with coleoid tentacles on the lower portions of their faces. They had no vocal cords and instead communicated by telepathy. There seemed to be no visual distinction between male or female Ood, which would suggest that they were hermaphrodites. (REF: Doctor Who: The Visual Dictionary) On the other hand, when Donna called an Ood "it", the Doctor corrected her, saying she should call him a "he". (DW: Planet of the Ood) 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. (DW: The End of Time)
The Ood had two brains; a forebrain in the head and a secondary hindbrain connected by an umbilical cord-like connection to their faces, usually held in their hands. (DW: Planet of the Ood) The forebrain did much of the thinking and stored the telepathic sensors. The hindbrain processed memory and emotions, leading to mental instabilities when removed. (REF: Doctor Who: The Visual Dictionary (2010)) The hindbrain also allowed them to communicate with the Ood Brain. (DW: Planet of the Ood) The Ood Elder had a larger, external forebrain. (DW: The End of Time)
Telepathy and mental abilities
The Oods were originally a hive mind led by the Ood Brain, using their hindbrain to "sing" with each other. (DW: Planet of the Ood) When in the servitude of Humans, their hindbrain 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. (DW: The Impossible Planet) 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". (DW: Planet of the Ood)
History
The Ood were a peaceful herd-race originally. In the 39th century, 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 hindbrain 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. (DW: Planet of the Ood)
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. (DW: The Impossible Planet)
In 43K2.1, the Doctor and Rose Tyler came to Sanctuary Base 6 on Krop Tor, where the Ood were used for menial labour. Unlike the Humans or the Doctor, the Ood knew of prophecies regarding the Beast which the expedition would later discover. (TS: Tardisode 8, Tardisode 9, DW: The Impossible Planet) 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. (DW: The Impossible Planet) 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 honours. (DW: The Satan Pit)
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. (DW: Planet of the Ood)
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 a mysterious face and of time bleeding. (DW: The End of Time) With this, they sent Ood Sigma to 2059 to contact the Doctor. (DW: 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 investigate. The Ood watched him and when he finally began to die, they telepathically sang to him. (DW: The End of Time)