Doctor Who secretly revealed the Ood attempted to invade Earth - but were repelled by an unknown incarnation of the Doctor. Since its relaunch in 2005, Doctor Who has introduced countless new alien species. The Ood - introduced in the episodes "The Impossible Planet" and "The Satan Pit" - were originally portrayed as a slave race, although in the end David Tennant's Ninth Doctor imprisoned them from their enslavement.
The Ood are a fascinating and unique race, possessed of not one brain but two - the second hindbrain actually held in their hands, connection by a sort of umbilical cord to their faces. This brain structure unfortunately makes them particularly vulnerable to psychic reprogramming, which was how humans transformed them into slaves by the 39th century. Telepathic beings like the Beast could easily take control of the Ood's hive mind, turning them into mere puppets to serve its will. And, intriguingly, one episode of Doctor Who - Matt Smith's introductory story "The Eleventh Hour" - hinted some alien force attempted to use them to invade Earth at some point in the 20th or 21st centuries.
"The Eleventh Hour" ends with the Doctor calling a race known as the Atraxi back to give them a scolding. He asks the Atraxi a series of questions, inviting them to plug into the Earth's information systems in order to answer them; the questions culminate in the Doctor's asking if this planet is protected. His final question causes the Atraxi to access secure servers from world governments and organizations such as UNIT, and they transmit images of the various aliens who have attempted to invade the Earth. "You're not the first lot to come here," he tells them as images of Cybermen and Daleks, Sea Devils and Sycorax are displayed before him. "And what you've got to ask is, what happened to them?" It's easy to miss, but one of the images displayed is of a race viewers did not know had previously invaded Earth - the Ood.
Of course, in reality, this is probably just a minor mistake in production. The team probably just went for cool images of iconic aliens from the history of Doctor Who, and they grabbed this tremendous one of the Ood looking threatening. But it nonetheless means there must have been an Ood invasion sometime in the 20th and 21st centuries, one that is presumably linked to the Doctor given he was the one who had repelled all the previous threats glimpsed by the Atraxi. What's more, it's reasonable to assume this happened during David Tennant's incarnation, because he was the first to encounter the Ood, and Matt Smith's Eleventh Doctor didn't react to seeing their image.
The interesting question, then, is just why the Ood invaded. They are a peaceful race, so it is likely they served as the pawns of a greater force, some psychic presence akin to the Beast or the Great Intelligence. Hopefully, sooner or later, Doctor Who will provide an answer, probably not in the TV series itself but in one of the spin-offs, perhaps by Big Finish.