Ood

The Ood, or Ood-kind as they called themselves, were a gestalt race of humanoids originating from Ood Sphere.

Biology
The Ood were a humanoid species with coleoid tentacles on the lower portions of their faces. They have no vocal cords, instead communicating telepathically. 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 hold in their hands). The hindbrain enabled them to connect to a single large Ood Brain, which seemed to coordinate the individual 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", cutting 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 change 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)
 * However, when Donna called an Ood "It", the doctor corrected her, saying she should call him a "he".

History
The Ood were a peaceful race, evolving in the cold climate of the Ood Sphere. 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 changes made to the Ood's physiology by Ood Operations meant that they were willing for this to happen to them. (DW: Planet of the Ood)

By the year 43K2.1, 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 the "Friends of the Ood" were lobbying for Ood freedom. (DW: The Impossible Planet)

When encountered by the Doctor and Rose Tyler, a group of Ood was part of the Walker Expedition. 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 formed the Ood on the base into its "Legion". (DW: The Impossible Planet)

The Ood were defeated when Danny Bartock, the expedition member in charge of them, broadcast a telepathic flare which reduced their field to "Basic Zero", creating 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 been freed of the Beast's control, and all of them perished. All of them were given posthumous honours. (DW: The Satan Pit)

In 4126, the Doctor and Donna Noble visited the Ood Sphere and Ood Operations, and decided to learn the origins of the Ood and what turned them into a slave race. At the same time, the "red-eye" disease was becoming more and more prominent. After discovering that the disease was caused by the Ood Brain's subconscious reasserting itself over the Ood (thanks to the efforts of one member of the Friends of the Ood who had infiltrated the company), the Ood were set free and allowed to roam their homeworld in peace. (DW: Planet of the Ood)