Oswin Oswald was one of the many splinters which resulted from Clara Oswald stepping into the Doctor's timeline. She once claimed that the first boy whom she ever fancied was called Rory, but quickly admitted that "actually, she was called Nina. I was going through a phase."
At some point, Oswin joined the starliner Alaska's crew as Junior Entertainment Manager to see the universe. When the Alaska crashed on the Dalek Asylum, Oswin escaped down a shaft into the lower levels of the Asylum, where she was found and captured by the Dalek inmates. Unlike the other crew members of the Alaska, who were partially converted by the nanocloud into Dalek puppets, Oswin was fully converted due to her exceptionally high intelligence, something they valued over exterminating her. Due to the horror of what had happened to her, Oswin created a dream world for her human mind to dwell in, to forget the truth. Useless to the Daleks, she was locked in the Asylum. Under this life, Oswin assumed that she was surviving on soufflés in the Alaska. She also hacked into the Asylum's fully-automated systems, heavily damaging it and reducing it to a ruin.
A year later, Oswin aided the Eleventh Doctor, Amy Pond and Rory Williams in finding safe passage through the Dalek Asylum. She later wiped all the information about the Doctor from the Daleks' Pathweb at one point to save him from the Daleks in Intensive Care. The Doctor attempted to rescue her until he found her and discovered she was a Dalek. The Doctor explained this to Oswin, who first nearly gave in to her Dalek conditioning by attempting to exterminate the Doctor, then broke down over what she had become.
Oswin lowered the Asylum's defences so the Parliament of the Daleks could destroy the planet and asked the Doctor to remember her as the human she once was, with her last words, "Run, you clever boy... And remember." When Amy, Rory and the Doctor escaped back to the Parliament, they found out that Oswin had erased the Doctor from the memories of all the Daleks in the Parliament as well as the Dalek inmates in the Asylum. (TV: Asylum of the Daleks)
When the Doctor later encountered Clara Oswin Oswald, he noted that she had a similar voice to Oswin but failed to make any connection between the two until he saw the barmaid's full name on her tombstone, setting him on the path to find the original Clara. (TV: The Snowmen) Following his meeting her, the Doctor recalled Oswin when Clara demonstrated her new hacking skills and chose the username "Oswin". (TV: The Bells of Saint John) The Doctor later brought up Oswin to Clara when questioning why he kept meeting her but Clara later lost her memories of this when the timeline reset. (TV: Journey to the Centre of the TARDIS) She regained them when climbing through a wrecked future version of the Doctor's TARDIS, allowing her to deduce that she had to enter the Doctor's timestream. (TV: The Name of the Doctor)
Oswin's actions had a profound impact on the New Dalek Paradigm whose war effort ground to a halt for centuries as they attempted to make sense of the information she had deleted. Ultimately however, her efforts were later undone when the Daleks harvested information about the Doctor from Tasha Lem. (TV: The Time of the Doctor, PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)