Great Old One

The Old Ones, sometimes known as the Great Old Ones, were beings from before the universe, according to the Doctor. (NA: White Darkness, All-Consuming Fire, MA: Millennial Rites)

Overview
The Great Old Ones were unimaginably ancient creatures that were preserved in Humanity's race memory as their worst nightmares. They were conceived as creatures that spread evil, chaos and destruction wherever they went. (BFA: The Roof of the World)

Quantum mnemonics was known to be the language of the Old Ones, as they represented physical laws of the Universe that preceded the current one. This allowed them to rewrite reality on the most fundamental of levels. (MA: Millennial Rites)

History
Following the demise of the last universe, the Old Ones passed on into this one in which they discovered they had god-like powers with which they manipulated the universe. (MA: Millennial Rites)

Ancient carvings show that the Old Ones escaped some great cosmic disaster in order to flee into the Time Vortex where they became discorporate entities thus leaving their bodies behind. They have sought to return ever since. (NA: White Darkness)

At one point they attempted to enter into the higher dimensions by using the Palace of the Old Ones to power a gateway. One of their number, known as Valdemar, discovered that his kind were unprepared for the perceptions of this new reality and were being driven mad by it. This breach would have destroyed the universe had Valdemar not sacrificed himself to seal the breach. Legends would spread across the galaxy that he was a dark god that nearly destroyed the Old Ones though the truth was far different. (BBC: Tomb of Valdemar)

The Pharoah Amenhotep discovered them at one time on Earth and attempted to harness their power for himself. However, he came to realise that they were beyond his control and lured them into a white pyramid with a structure of infinite reflections where the creature's evil were to be trapped forever. This trap succeeded and kept the world safe from their evil. (BFA: The Roof of the World)

List of Old Ones

 * Azathoth
 * Cthulhu
 * Dagon
 * Hastur the Unspeakable
 * Several Lloigor, including the Animus
 * Nyarlathotep
 * Shub-Niggurath
 * Yog-Sothoth
 * Magog
 * Na
 * Rag
 * Ok

Possible Old Ones

 * Though not an Old One itself, the Nestene Consciousness is the child of Shub-Niggurath. (PDA: Synthespians™)
 * Though not explicitly described as such, the god-like Golgoth seems very clearly reminiscent of the entities of the Mythos. (DWM: The Cybermen: The Dark Flame)
 * An unknown Old One was said to be within the ship Infinite as a power source. (DW: The Infinite Quest)

Behind the Scenes
The Old Ones originated out of the Cthulhu Mythos, a shared universe based on the writing of H.P. Lovcraft and to an extent, some of his contemporaries. A few characters and concepts in televised Doctor Who, such as Sutekh (though based on an actual ancient Egyptian god) and the Fendahl, had shown the apparent influence of the Mythos. However, not until the novels White Darkness, All-Consuming Fire and Millennial Rites did beings explicitly referred to as Old Ones appear in fiction set in the Doctor Who Universe.

The latter two books took known Doctor Who enemies, such as the Animus and retconned them into having links to the Mythos. The Taking of Planet 5 revealed that the Cthulhu Mythos also existed in the form of fiction in the Doctor Who Universe. Though the established Cthulhu Mythos and the Doctor Who version have some differences (the idea of the Old Ones originating before the universe originates in Doctor Who novels, for example), stories set in the Mythos frequently contradicts itself.