Having the same actress play different characters has precedent in the modern doctor who. Watch the battle of Canary wharf, Freema Agyeman who would later play a companion (Martha Jones) is seen playing Adeola, an employee of Torchwood. It was addressed a few episodes later when Martha said she had a cousin who worked at Canary Wharf.
Couple of things that have me scratching my head
-Oswin looks directly at the camera in her final shot. Clearly it's scripted but to what end? it'll probably make more sense when we understand her relationship to Clara Oswin.
-How much did Oswin remove from the collective Dalek memory? Forgetting one person, okay bit of a stretch. But not knowing to fire on a time lord jumping out of a TARDIS on their own ship? Was she able to remove the entirety of the time war from their memory?
I like the idea that Oswin and Clara are the same person from different points in her own time line. Given the guilt the doctor's been experiencing over companions since the stolen earth about turning companions into weapons. Leaving the Ponds behind to protect them while there still alive. If Oswin/Clara are the same the Doctor could be trying to alter her timeline to prevent her from being on the Alaska when it crashes.