One day Three, after a round wih the Master, rescues an Ood from his TARDIS. Like he can’t move his own TARDIS and take him back the Ood-Sphere, he just get him to UNIT and tries to keep it hidden for the rest of people.
But one night, it escapes, and, how it’s conditioned for serving, it begins to clean up, and that’s how the Brigadier founds him cleaning his office at the next morning. Lucky for the Ood, The Doctor appears before the Brigadier shoots it (it’s the Brig, of course draw his gun is the first thing he thinks when he founds an alien classifying the files on his desk in alphabetical order).
And that’s how the Ood becomes the unofficial caretaker of UNIT.
Jo loves the Ood, and the Ood loves the nice “Miss Josephine” (Miss Jo, after Jo asks it ten times don’t call her Miss Josephine), and even she knits for it a cozy beanie and a scarf, because it’s a very cold winter.
The soldiers don’t know how to treat it at the beginning, but soon it becomes one of them, and you can see them trying to teach it how to play football on the break time or sharing coffee making skills with Benton.
Years pass, The Doctor comes and goes from UNIT, and one day, meanwhile he’s making a visit to Kate, Twelve finds out that the Ood is still there, and that he’s completely forgotten about it. He offers take it back to its planet, now he can, but the Ood refuses, it’s happy there, UNIT is its home.