Ok, for the record, we don’t know the context of what is happening, BUT, Stephen Moffat himself has revealed that Osgood is indeed coming back to Doctor Who in the upcoming series:
Steven Moffat, lead writer and Executive Producer, said, “Osgood is back, fresh from her recent murder at the end of last series. We recently confirmed that Osgood was definitely dead and not returning – but in a show about time travel, anything can happen. The brilliant Ingrid Oliver is back in action. This time though, can the Doctor trust his number one fan?”
So…that’s interesting…but it gets better, cause the actress who plays Osgood, Ingrid Oliver, was just as surprised!
“As every actor who’s worked on Doctor Who will tell you, there’s always the secret hope you’ll get the call asking you to come back. To actually receive that call is both unexpected and brilliant. The word ‘honour’ gets banded about a lot, but it really is, it’s an honour. Especially because I was so sure Osgood was a gonner after the last series!”
Again, this is Doctor Who, where ANYTHING and ANYONE can show up and not be a continuity break. We’ll just have to patiently for the episodes to return so we can learn the “truth”.