The Ood require a translator device, a small sphere connected to their "mouths" by a tube, to facilitate speech between them and humans. The tube was originally connecting their external brains to their body, but to use the creatures, far future humans would amputate the brain and instead fix the translator sphere where the brain used to be. There appears to be no gender differentiation among the Ood, and they say they require no names or titles as they are "one", but they do have designations such as "Ood 1 Alpha 1". The Ood are empaths, sharing among themselves a low-level telepathic communication field, rated at "Basic 5" (with "Basic 30" being the equivalent of screaming and "Basic 100" meaning brain death). When reaching out with their telepathic fields, it can be heard as singing. According to the Official Doctor Who Annual 2007 the Ood live on a planet in the Horsehead Nebula where they were governed by a Hive Mind but it was destroyed by Human colonists. According to the monster book Creatures and Demons, published in 2007, it says they come from the "Ood Sphere", close to the "Sense Sphere" planet, home to the Sensorites, who share a similarity with the Ood. With no hive mind the Ood offered themselves to the Human colonists and became a slave race.
When encountered by the Tenth Doctor and Rose Tyler in 'The Impossible Planet', a large number of Ood accompanied a human-led expeditionary force on the planet Krop Tor, orbiting a black hole. 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 his "Legion". While possessed, the Ood 'zapped' and killed two human security guards by throwing their translation spheres at them.
The Ood were defeated when Danny Bartok, the expedition member in charge of them, broadcasted a telepathic flare which reduced their field to "Basic Zero", creating a "brainstorm" which caused them to collapse. However, the 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. It is unknown if the Ood on Earth, which were seen in 'The Impossible Planet's TARDISODE, and on their home planet of the Ood Sphere, were influenced by the Beast's control, or not.
On the return of the Ood in the 2008 story 'Planet of the Ood', it was revealed that they are not, in fact, born to serve, but are an enslaved race, with the translation spheres actually replacing their hind brain which had contained their individual personalities. The Doctor aids and successfully frees the race by releasing the main Ood brain, which links all Ood with a telepathic link (which may be why they were before only able to communicate at basic 5). Before this time it had been sequestered away from the Ood for 200 years by Ood Operations, the corporation that processed the Ood slaves. Over that time the brain adapted (assisted by the gradual lowering of its suppression field by a Friends Of The Ood activist who had infiltrated Ood Operations), allowing it to influence the Ood's actions. It made some Ood feral and vengeful (causing their eyes to glow as possession by the Beast did), while directing another to genetically re-engineer the head of the slave trading company into an Ood. Once the Hive brain was freed, the Oods' song could be heard throughout the Human Empire, by Ood and Human alike. After this, all Ood were freed and sent back to the planet of the Ood.