O (The Furies)

O was a Proto-Time Lord clone of River Song.

She was part of a second batch of clones created by Madame Kovarian after River failed to kill the Eleventh Doctor at Lake Silencio. Like River and her fellow clones, O was raised on bedtime stories about the Furies and trained to be an assassin. She grew up considering Kovarian to be her mother and believed Kovarian's promise to give her a proper name after her first mission. However, as soon as she realised River wasn't responsible for H-One's death, she agreed to join forces with River against Kovarian. Kovarian unsuccessfully tried to sway her by saying she was planning to name her Flumen. After Kovarian was defeated and the Doctor was saved, O was taken by the Doctor to the nearest galactic club card trading outlet, on River’s request. (AUDIO: The Furies)

Behind the scenes
Like all of her siblings, O's name is a reference to water, along with her sisters H-One and H-Two, her name is a pun on H20, the chemical formula for water. A water molecule is composed of two hydrogen atoms (H-One and H-Two) and one oxygen atom (O).