Top 10 Theories About the New Doctor Who Companion
6. Oswin is the heart of the TARDIS. Her sudden and brazen kissing of The Doctor as Clara, as well as her remarking that the TARDIS is smaller on the outside rather than bigger on the inside, could be clues to this. It's sort of unlikely, though, knowing what we do of the way the TARDIS works. She could be the heart of another TARDIS.
5. Oswin is actually the computerized little girl CAL from "Silence in the Library." Both are brunettes, and CAL would have enough information about The Doctor and River Song (spending her afterlife caring for CAL in the Library's files) to know to coax The Doctor with mention of the Ponds. Possibly she is using projection into flesh-like bodies to leave her digitized safehouse.