It isn't like we were allowed constant access to the castle as well. Though, I believe it is entirely possible to conceal it as well, perhaps depending on how he was being transported, and the timing.

If Tatara and the Sage's assistant had noticed, either one would have certainly said.

[ Jing Yuan believes it is entirely possible that Oz has been before they arrived, though he is also open to the idea that he could have very well been transported under their noses into it, or a whole list of other possibilities. ]

...That said, what do you mean by that, exactly?