In an episode full of tragic moments, the murder of Osgood in Death in Heaven was among the most brutal. The Master/Missy vaporised the UNIT scientist/knitware enthusiast/audience surrogate simply to mess with the Doctor. But now, somehow, she's back and starring in The Zygon Invasion. But how?
Let's call our first theory "Ozgood", as it's probably the most likely. At one point in the 50th-anniversary special the Day of the Doctor (the one with Matt Smith, David Tennant and John Hurt in it) a Zygon takes Osgood's form, starts imitating her and is really mean to her. Then later in the episode, in order to broker a peace deal, the Doctor wipes everyone's memory so humans and Zygons can't remember if they're humans or simply Zygons pretending to be humans.
Perhaps the Zygon's memory never came back, and she has been living as Osgood since then. Perhaps, with the original human Osgood dead, Ozgood is only now recalling her origins, a kind of sleeper cell. Perhaps, perhaps, perhaps. As Moffat himself said: "The brilliant Ingrid Oliver is back in action. This time, though, can the Doctor trust his number one fan?”
Sometimes the simplest solution is the correct one...even if that "simple solution'"involves traveling back in time to before Osgood was killed. As Moffat boasts, "In a show about time travel, anything can happen." If it is a case of simply going back in time to pre-death, the Doctor is unlikely to save her outright – paradoxes and all that. The situation might be similar to his relationship with River Song, when he knew all along that she was doomed. Bummer.
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Now don't laugh. This is the RadioTimes.com pet theory, because it's both ridiculous and founded in Doctor Who lore, the two things that get us up in the morning.
"Vaporisation" special effects are nice and vague and are often revealed to be a disguised teleportation. The entire first episode of series nine depended on this, with Missy, Clara, the Doctor and the Tardis all escaping the Daleks by faking their own disintegration.
It is possible that Missy’s strange smartphone/weapon is a modern update of the Master’s old Tissue Compression Eliminator. This used to shrink victims down to doll-size through “matter condensation.” An improved version would presumably mean better shrinkage. Could a tiny, subatomic Osgood still have been alive and well, living in Missy’s iPhone (and somehow have since been returned to full size)?
These are the questions that need answering.
Look, she's not, but let's dispense with this now because we know someone is feverishly tapping this theory into a Doctor Who forum.
Essentially: Clara isn't just Clara, she's every Clara. It's all in her. In order to save the Doctor from the Great Intelligence in the Name of the Doctor, she scattered herself throughout time, constantly cropping up in the Doctor's life and saving him from danger. Every time she had the same face, but her names varied slightly. The Clara we know best has the surname Oswald. The very first version of Clara the Doctor met (well, sort of, it's complicated) was called Oswin Oswald. And Osgood is called Osgood, which is a bit like Oswin/Oswald.
It should be noted that this theory is wrong. Clara may have changed names, but she always had the same face. (Except when she was a Dalek, but let's not get into that). Ingrid Oliver and Jenna Coleman do not have the same face. QED.
Nope. Just nope. She may dress up as her favourite Time Lord, but she isn't one herself.
Osgood's sister is mentioned in the Day of the Doctor. Apparently she considered her sibling 'prettier', but Osgood has always had a bit of an inferiority complex. It's possible that this sister is a twin, and as Osgood is a surname, she would also be called Osgood. This theory gets extra points for sounding like an off-cut from a Dallas episode.
Minor spoiler warning – Here's Steven Moffat's episode guide for the second half of the two parter, The Zygon Inversion:
The future of planet Earth is sealed in a box in Unit's back archive, and only the Doctor knows what’s inside. With Unit under Zygon control, and Clara lost, the Doctor and Osgood find themselves fugitives in a London where no one can be trusted - but the wily old Time Lord knows there is one last hope for peace. Because that box in the black archive isn’t any old box. It’s an Osgood Box!
What is an Osgood Box? Is it a box with spare Osgoods inside? A box that turns people into Osgood? Osgood's lunchbox? Whatever it is, we wouldn't be surprised if we ended the two parter with more Osgoods than we began.
Moffat moves in mysterious ways. For every time the fans guess where he is going (Missy/Master) there are a thousand moments of 'huh'? Yes, maybe Osgood is a ganger clone, or trapped in Gallifrey's pocket dimension, or a hitherto unseen pre-regeneration of the Master, but the reason for her return is as likely to be something completely new.
All we know for sure is we're glad Osgood is coming home.