The Circle must be broken.
The Ood (or, as they sometimes called themselves, 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 have no vocal chords and instead communicated by telepathy. They were originally a hive mind led by the Ood Brain. This kept control of the two brains in each Ood (a forebrain embedded in their body, and a secondary hindbrain connected by an umbilical cord-like connection to their faces, usually held in their hands). The hindbrain enabled them to connect to a single large brain, which coordinated individual Ood. (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. 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)
There seem to be no male or female Ood, which would suggest that they are hermaphrodites. (Doctor Who: The Visual Dictionary) Despite this, when Donna called an Ood "it", the Doctor corrected her, saying she should call him a "he".
History
The Ood were a peaceful race originally. In the 3800s, 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 modified by Ood Operations and replaced with their translator globes. With their connection of the Ood Brain severed, they followed the orders of Humans.(DW: Planet of the Ood)
By the year 43K2.1, when the Doctor and Rose encountered the Walker Expedition, 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) 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 Ood were set free and allowed to roam their homeworld in peace. (DW: Planet of the Ood)