The Ood Brain was a huge living brain that under natural circumstances controlled the connection between individual Ood, facilitating their Hive mind. It was capable of telepathically reaching Ood over the distance of three galaxies through an operatic song. While normally passive, it was capable of anger and rage which it expressed through the Ood.
A few centuries before the 42nd century, humans discovered the brain beneath the Northern Glacier on the Ood Sphere. By 4126 it was contained in warehouse 15 of Ood Operations, surrounded by a telepathic barrier so it couldn't communicate with the Ood. It was kept alive, even though it had to be contained, because without it the Ood would die. The neutralisation of the brain, together with lobotomising the Ood, allowed them to enslave Oodkind. Dr Ryder, an agent working for Friends of the Ood, managed to infiltrate Ood Operations and after years of patience came into a position where he could covertly lower the intensity of the field, which first manifested itself as Red-eye and later resulted in a full-blown Ood rebellion. The Tenth Doctor eventually shut off the barrier, freeing the Ood from slavery, forever. The Doctor described the Ood Brain as having expressed itself through the Ood in three stages: its anger at being imprisoned was red-eye, its rage at the humans was the rabid Ood, and its patience and mercy was focused on Ood Sigma. (TV: Planet of the Ood)