We know Coleman arrived earlier than expected. It was previously discussed the new Companion would make her first appearance in this year’s Christmas Special and that may still be the case. At the Season 7 premiere I attended in NYC, Executive Producer Caroline Skinner said casting Coleman as Oswin in the episode was just a last minute genius idea by Moffat. It was certainly a pleasant surprise but it doesn’t necessarily mean that Oswin and the new Companion are one in the same. We’ve seen actors utilized on the show (and Torchwood) more than once.
FOR f***'S SAKE, MOFFAT.