Aaaand it's occurred to me that nobody ever actually set eyes on Oswin; so her appearance didn't matter as far as the story was concerned. I believe we will see Coleman again before she becomes a regular companion.

What's interesting about this is that the audience knows what Oswin looks like, but the Doctor has no idea. So when he runs into his new companion, there's every chance that the audience will instantly know that she's the girl who got converted into a Dalek but the Doctor won't.