I think Jack is making up new words.
He has been communicating with us his whole life, through cries and coos, hand gestures and signs, and now words and sounds. For a long time Jack has been telling us no. It started with "unh-unh" and has now progressed to a very clear "no-no."
It's been a little harder to understand when he means to say, "yes." We taught him to nod his head for yes and that has worked pretty well. But now when he answers a yes or no question, the answer may be "oh." Now, I know, you are thinking that this sounds a lot like no. I thought this too. But when he seems to mean yes, he is saying, "oh." I'm not sure where this came from. Maybe it's his version of "ok" or the sound he thinks is "yeah."
So, I think Jack has figured out something else to say since the word yes may be too difficult to say. It has just taken his father and I a few days to realize it.
A note on Choo-choos:
As I sit here typing, Jack is emptying my basket of his clean laundry of anything that has trains on it. He has taken out the Thomas T-shirt and the two pairs of Thomas pajamas. He has also pulled each of them over his head and face, I think, in an attempt to put them on.