Ood Sigma

Σ (Sigma) was the personal Ood of Klineman Halpen, manager of Ood Operations. He was the most important Ood. He dosed Halpen with Hair tonic as he was balding. Sigma travelled with Halpen to Ood Operations. Sigma remained by Halpen's side even when all the other Ood were infected with Red Eye. Sigma led the Doctor and Donna Noble to the room containing the Ood Brain.

There Sigma revealed to Halpen he had been dosing him with a liquid that turned him into an Ood using the hair tonic; while the red-eye Ood had been 'contaminated' by the Ood Brain's anger, Sigma had become the focus of the brain's patience, patiently transforming Halpen into an Ood himself. Sigma declared that the Ood would look after Halpen. As the Doctor and Donna departed, Sigma assured them that they would always be remembered as friends of Ood-kind, and would have a part in the song for as long as the Ood existed. (DW: Planet of the Ood)

This particular Ood can be identified with the large Greek letter "Sigma" (Σ) on the right side of his uniform.